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Knocking Noise at Start Up

2000 Blazer 4wd, automatic, 122000 miles

In the last couple of weeks, if the Blazer isn’t driven for 3 or more days, there is a slight knocking noise for 15 to 20 seconds after start up. Oil level and pressure are fine. I’m guessing it’s the lifters, but I’m open to any suggestions.


Ed B.

How long since the last new oil filter? Any oil pressure gauge or light info would be helpful.

Lifters is a reasonable guess. At 122K, most everybody starts making a little noise. It could be just an accessory, like the water pump, power steering pump, alternator starting to mis-behave. If you have access to an engine stethoscope, you might be able to narrow it down a bit based on what part of the engine the sound is coming from. I’d check all the fluid levels including the automatic transmission and power steering pump fluid just to make sure one of them being low is not the cause.

Is it a “tap, tap, tap” kind of noise or is it more like a heavy “thump, thump, thump…”

Another question would be: Does it happen with every start? A drained down lifter or two would not happen every time due to the engine’s camshaft stopping at various positions. Another possibility is piston slap but that would likely take a little longer to go away as the pistons heat and expand. A little piston slap at startup is harmless as is a little temporary loose valve lifter noise.

Four months and 1300 miles since last oil change. Oil level ~3/4 full, oil pressure normal (40 psi cold, 20 psi warmed up at idle).

All fluid levels good, alternator, ps, and ac quiet. Noise did not occur after sitting overnight. Noise duration is too short to track down.

Noise is closer to a light tapping, definitely not a heavy thump. Similar to an exhaust manifold leak, but not as loud.

Noise does not happen every time. More likely to occur after the truck has been sitting a few days.

Ed B.

I’ll guess at valve noise. Does it use hydraulic lifters?

I’d also not actually rule out a very small manifold leak - one that mostly seals up almost immediately with some heat expansion.

Lifter almost for sure … not an issue at this point … could get worse in another 100k or so. (Also bet it puffs a little smoke out the exhaust when started after overnite or longer)

Probably the lifters and not a big deal. You might try a different brand of oil filter. Some don’t have a very good ‘anti-drainback’ valve and the pump has to refill the filter before you get any oil pressure. My 94 used to make a little noise at startup if it sat overnight. I switched to Wix filters and it doesn’t anymore.

Offhand, it sounds like a valve lifter rattle but the oil pressure also appears to be a bit low to me at 20 PSI on a warm idle. (The lower oil pressure would not cause a lifter rattle. A lifter rattle would be caused by a faulty lifter; either a mechanical fault or grunge which is causing it to bleed off.)

I drove it into work today and kept a close eye on the pressure gauge. This is the dash gauge so take the readings with a grain of salt.

Cold idle at start up - 50 psi
After 10 minutes driving, idle and cruise - 30/45 psi respectively
Fully warmed up, idle and cruise (40-45 mph, 1300 rpm) - 20/40 psi

No rattle at start up after sitting overnight. I’m more curious than concerned about the rattle at this point. It may be time to change from 5w-30 to 10w-30 oil.

Thanks for all the replies,

Ed B.

I don’t know what the specs are for this engine, but your oil pressures seem perfectly acceptable to me, especially for a 12 year old engine. Try a different brand of oil filter. Maybe a full synthetic oil would benefit you too, as they definitely flow better on a cold start.

I’m going to go with sticky lifters (I’m an optimist). The other possibility is a bearing knock. When you shut an engine down, normally oil stays pretty well between the main bearingas and the crankshaft due to being entrapped between the surfaces, keeping some lube there for startup, but if there’s a lot of wear and the spaces have grown, the oil could be running off and you could be starting with a basically dry bearing. It’d take 15 or 20 seconds for the spaces to get refilled by the pumped oil.

Oh, and that form of wear would also reduce your idling pressure.

I’d try a heavier weight or a synthetic. Perhaps one of them will stay in the spaces better.

20 PSI at idle (650-750 RPM?) with a hot engine sounds just fine to me. If it didn’t get much above that when you accelerated, that would be worrisome and indicate excessive overall bearing wear. Of course it only takes one damaged bearing to knock.

I had a 2002 Chevy Blazer. There is a defect with the intake manifold gasket failing that lets coolant leak down onto the lifters and into the engine. I had the exact same tapping but I also had coolant disappearing which I thought was an external leak. Anyway the tapping was caused by the coolant mixing with the oil thinning it out or maybe dripping down on the lifters. I stopped driving it and replaced the intake manifold gasket then changed the oil and ran a cleaning oil in the engine for a few hundred miles then changed the oil again. It fixed the problem.