Subaru Outback Intermittent Knocking

The car in question is a 1998 Subaru Legacy Outback. Just had the heads resurfaced and new gaskets, and timing belt. Runs great. Acceleration is good, stays cool. However, it has developed an intermittent knock/tapping. it sounds like its coming from the top end of motor. I added Marvel Mystery and the noise left for about a week but came back. Is this the beginning of the end? Or could it be a small problem involving the lifters/followers/valves? I’m prepared for the brutal truth! Thanks!

When is it most likely to tap? Right after start-up? After it warms up? At idle? At tip-in (just opening throttle)? When accelerating? Coasting?..

Definitely after it warm ups…then at idle and acceleration(me working the throttle body). don’t notice it when on the highway or even driving in light traffic. Thanks for response!

Have you tried a higher octane gasoline?
Milling the head can increase the compression to a point where preignition introduces itself. That can take the form of pinging and/or knocking.
Take some compression readings and post them here.

Thanks so much!! will do!

Double check the timing and try the next best gas, low to mid or mid to premium. What conditions can you replicate the knocking, going uphill etc.?

If there’s a way to retard the ignition timing a tad, might be worth an experiment. And it might pay off to have a shop inspect that the valve clearance is within spec and if hydraulic lifters, that they are working properly.

A small exhaust manifold gasket leak can make a ticking noise.

Good point @insightful … OP could use a piece of garden hose as a stethoscope to see if the sound was coming from the exhaust manifold area.

Man! Preciate you guys. I feel like a patient getting great advice from a panel of doctors. Exhaust manifold gasket? never thought of that. I am definitely curious about the valve clearance and lifters working properly…and the timing…which was messed with as a new timing belt was put on??? I am taking all of these ideas to my mechanic and have him consider all. Thanks so much fellas.I will keep you posted!