Tapping sound that diminishes after warm up

Car & History:

  • 99 Jetta, 2.0 AEG engine, manual. 86,000 miles. Bought at 70,000 miles 3 years ago.
  • low oil warning once in 2013 maybe. since been using 5w-40 synthetic and used Ceratec additive once. Been doing regular oil changes.
  • replaced original fuel filter at 80,000 mi
  • gaskets have been going. Replaced thermostat and gasket. Replaced leaking oil cooler gasket. Valve cover gasket has been leaking for at least 1 year. Will replace.
  • Replaced timing belt, tensioner, water pump, and serpentine belt at 84,000 miles.
  • Flushed radiator at 84,000 miles.
  • All break rotors and pads at 80,000 miles
  • Replaced battery at 75,000 miles
  • Gas mileage: 26mpg


  • the sound is similar to a tapping. This has been going on for a year or more.
  • very noticeable and loudest when cold
  • worse in the lower gears
  • diminishes once warmed up (5 minutes)
  • happens only under load - so 5th gear and letting off the gas pedal (coasting) it’s not noticeable.
  • JUST TODAY: 20 degrees F, and wouldn’t start on first try. Cranked for 6 seconds probably. Second time I turned the key, it started no problem.

Things of note:

  • By peaking behind the engine, it looks as though the oxygen sensor wire covering (pre cat) is getting a little frayed at one spot.
  • When I bought it I noticed new spark plug wires.

My thoughts:

  • Exhaust manifold gasket (to the head) is bad. So when it gets warm the bad gasket expands and stops the noise.
  • It’s not the valve lifters because the noise goes away when warm and not really heard when gas pedal isn’t engaged.
  • Replace the O2 sensor?


  • 3 audio files. I put my car under my hood and went for a drive. You can hear when I engage the clutch, the ticking sort of goes away.
  • Pictures of the frayed O2 sensor wire

Thank you for bearing through this and I appreciate all ideas!


Could be a cracked exhaust manifold.

I have experienced the exact same problem on my 1999 Acura CL 2.3, 4 cyl. When cold I can hear, what I assume, are the lifters. For the last 2 years or so I am adding a bottle of Lucas oil additive with each oil change and never heard the lifters making this noise again, even in the winter when it is below freezing. For me, the fix was immediately after adding this Lucas stuff.

I have experienced the exact same problem on my 1999 Acura CL 2.3, 4 cyl. When cold I can hear, what I assume, are the lifters.

This contradicts your previous statement, “It’s not the valve lifters because the noise goes away when warm and not really heard when gas pedal isn’t engaged.”

Edit: Sorry, I confused comments.

Anyway, +1 to exhaust manifold being suspect.

I don’t know where I contradicted myself. I described MY experience with my car that made very much the same noise as heard in the audio files. Maybe, just maybe, the noise the Jetta makes goes away because of better oil flow throughout the engine when hot.Pushing down the gas pedal probably raises the engine noise and drowns out the other noise.

hey, a bottle of Lucas costs $10. Replacing the manifold a few hundred. What’s the damage trying it?


I guess we could argue over the use of the word “problem”, but I would think my description made it clear what I was trying to say.

Like I said before, I had the same problem (oops symptoms) and used the method described and it helped me get rid of my problem.

Mea culpa, @kurtfrommd , I confused yours and OP’s comments. (red face)

My hearing is not very good (60% hearing loss and Tinnitus) and combined with PC speakers it’s difficult for me to sort audible issues out without having hands on the car, but it comes across as a valve train clatter to me.
Valve lifters can certainly quieten down once an engine has been run for a while but an exhaust leak at the manifold can do the same thing.
If a can of SeaFoam was fed into the engine it’s possible an exhaust leak would show up shortly as SeaFoam will cause fairly heavy smoke (normal) after running for a bit.

As to the delayed start/ start on second attempt, that sounds like a loss of residual fuel pressure.
The system should maintain pressure while the car is not being used. If the pressure bleeds off the symptom can be exactly as you describe.

The most common cause of this is a leaking check valve in the fuel pump. The valve is basically a BB with a small spring which is designed to keep the BB on the ball seat when the engine is not running.
Usual causes of a leaking check valve is debris, broken spring (very rare), or a worn or pitted check ball or ball seat. The only cure is pump replacement.
This does not affect pump longevity or efficiency; it only presents a delayed start problem.

Hope some of that wild guessing helps.

OP said the oil warning light came on once in 2013. If she stopped the car right away and had the problem fixed, no foul. But if the car was run even for a few minutes after the light came on the engine could have damage. A can of Restore at the next oil change might quiet the engine, it will not cure anything, but can help a little.

I favor the lifter explanation as more common. I found that I could reduce this problem by switching to synthetic oil, especially for cold weather. In fact, the car called for 5W30 and I switched to 0W30 synthetic. The tapping went away about twice as fast.

@melott Makes sense! A 0W30 synthetic has a very low “pour point” and stays thin even at -40. It will circulate to the valve gear much quicker than a regular 5W30 mineral oil.

EXXON used to have a demonstration tape called “The Cold War” where they compared the ability to lubricate the valve gear at very low temperatures using different oils. The car used was a Ford Escort with a 4 cyl. OHC engine.

The 0W30 Mobil one performed the best, while a 15W30 mineral base oil resulted in camshaft and valve gear damage through lack of lubrication.

Both my cars have engine block heater and I plug in when I have to park outside in very cold weather.

Sounds like light piston slap to me. That is harmless. My Chev Cavalier had it and never got worse during 140k miles. When a piston heats up, it expands to make a tighter fit within the bore which ends the noise. Ignore and drive on.

This is another vote for valve lifter noise. My wife notices it on her Nissan Sentra and it’s always about the time that the oil needs changing. The noise always goes away with fresh oil.

if it is the exhaust manifold, it is what it is. could just be the manifold gasket? or the donut gasket between the the pipe and manifold (if it has one)

if it is an oil problem (and just to be safe) I would at the very least let it warm up and stop tapping before I put any load on it by driving in gear.

the other suggestions might just solve your problem. either way, it s low mileage for a jetta, I think, and worth fixing

Re: sometimes not starting… I agree with OK 4450. The solution is to make the fuel pump run enough to build up pressure. If it doesn’t start right up, turn the key to Off then to Run (not all the way to Start.) When in Run the fuel pump will run for a couple seconds. Turn the key to Start and the starter motor will turn over the engine. If the engine doesn’t start right away, try the key dance (Off to Run, back to Off) another time or two before turning it to Start…

Re: the tapping. It’s probably not hurting the engine. A little extra valve clearance or some piston slap will make this noise, which can be disconcerting, but no measurable harm is done.

You might do well to reconsider using the 5W40 oil and the additive. What oil is recommended in your owners manual? I doubt it’s as wide a range as 5W40. What does the manual say about oil additives? The wider range oil has more viscosity enhancer but less lubricity. The additive messes up the overall viscosity and lubricity even more.

First let me thank you for the best organized and most complete and concise post we’ve had in a long time.

I agree with the lifters as the most likely source. They’re hydraulic, they fill with oil to expand to take up the space between the cam lobe and the valve stem when the engine is running, constantly adjusting, and normally they retain their oil when the engine is stopped. But when they get old they can get sticky and worn and some can lose their fill and clatter until the oil warms up (thins) a bit, the pressure gets up, and the lifters (tappets actually… but a rose by any other name is still a rose) fill up again. Long term, the hammering on the tappet can cause damage, but it takes a long time and sometimes it isn’t worth changing the tappets on an old engine.

Re: the starting, I also agree that it may be the fuel line losing pressure and, if the pintle in one of your injectors isn’t seating fully, even leaking down. To leak down it needs not only the check valve to be leaking, but also a source for sir to get in… otherwise it acts like a straw with your finger over the hole… the line will retain the gas even if the pressure goes away. It isn’t serious, and if Shanonia’s recommendation works, you can even build that protocol (turning the key to ON a few times before turning it to START) into your morning starting ritual. I had a very old pickup that had this problem, and that’s what I did.

I still think it is an exhaust noise (cracked manifold ?). That noise almost goes away when you lift off the gas, lifters don’t.

Lifter noise for my VW diesel with a single marginal lifter does not go away in 5 minutes. Little oil warming takes place in 5 minutes, 15 or 20 minutes would be more realistic so I am doubtful that the problem is lifter noise. Also, depending on the lifter positions at engine stop, some of the time a lifter can become collapsed due to valve spring pressure while other times, the valve with a marginal lifter may be closed while the engine is stopped so there would be no spring pressure to tend to collapse the lifter. The OP might want to state if the noise happens every time or not.

It’s tough to really tell…but I also think it’s the lifters.