Subaru Flat-4 knocking

Greetings all,

I have a 2004 Subaru Legacy with a 5-speed that I purchased new in 2004. The car currently has 89000 miles on it. I have run Mobil-1 synthetic oil since 12k and all maintenance has been performed as called for by Subaru. Oil is changed between every 6000 and 7500 miles, but was changed every 3k before I switched to synthetic. (yes, probably overkill but I feel better doing it)

When the engine is cold, I get a knock from the right-hand side when it is idling (can slightly hear it), and can really hear it when the engine is under load, even if it’s just pulling out of the driveway at low RPM. I do not hear the knock if I just give it throttle when it is cold and do not place any load on the engine. This makes it very hard to pinpoint the noise under hood.

This issue has existed since approximately 40k miles and has progressively gotten worse. It is worse when the weather is cold, especially when the car sits out overnight and it is below 50 degrees. Allowing the car to warm up before driving makes minimal difference unless I leave it for 20 minutes or more.

I suspect the timing belt tensioner is going bad, but I’m afraid piston slap may also be an issue. What’s your experience on this issue?

For what it’s worth, I suspect I’m slowly losing a head gasket. Twice-weekly coolant checks usually come out okay, but every month or so I have to add about 6oz. to the recovery bottle. There’s no correlation to the amount of coolant loss versus short trips (7 mile commute to work) or long road trips (multiple trips typically totaling 2000 miles/month).

I don’t trust the local dealer (mis-diagnosed a bad slave cylinder as a failed clutch) and have not had luck finding a specialty Subaru shop in the area (central PA).

Any input is appreciated. Thank you!

Noises are hard to guess at since they’re pretty subjective. What may be a knock to you may be a rattle to me.
A faulty lifter or improper valve lash may come across as a rhythmnic rattle that may quieten down after a bit.
A piston slap is often a more erratic type of rattle and may sometimes appear to be worse on deceleration when the throttle is hit suddenly.

These noises often come across as more “tinny” and “metallic” in sound. A knock such a rod bearing will be deeper in tone and may come across as a sharper “rap” type of sound. A main bearing knock will often come across as a deep “thump” sound due to the engine design.

With only 89k miles you should not be suffering a bearing knock unless coolant is diluting the engine oil due to a head gasket fault. In this case, anything is possible.

If you want to rule out a center main bearing knock (the most common main problem) unplug the Number 3 plug wire and start the car. The car will run poorly of course but quickly note if the noise has gone away. If the noise disappears you have a center main problem; and that’s not good. Center main knocks are usually more noticeable at idle and very low RPMs.
Hope some of that helps.

Thanks for the info. Disconnecting the #3 plug wire and starting the car showed the noise is still there.

The valves are noisy on this car, but I don’t believe that’s what I’m hearing for this particular noise. A lifter could definitely be possible.

What’s the typical valve adjustment schedule? I don’t see anything in the maintenance booklet on it. My Subaru service manual does give the procedure, but does not call out a specific interval.

The general interval (and recommended one) is about 100k miles but this is something I take a serious issue with. Some other car makers such as Honda and Toyota also recommend 100k miles + valve lash inspections and this is seriously misguided IMHO: especially the part about checking valve lash by “listening to it”.
That’s beyond ludicrous as there is no way on earth anyone can tell what valve lash is by applying an ear to it.

Too noisy and over an extended time this can damage lifters, cam followers, and even cam lobes.
Too quiet and cylinder head valves/seats can burn out.

My opinion is that valve lash should be inspected and adjusted if necessary about every 30k miles at least; even more often if a problem is suspected.