148am

Piston slap makes sense based on this car burning oil when I ran Mobil 1 -- about 1/2 Qt in 1000 miles -- but not on dino oil (Castrol GTX, now Rotella). That oil was going somewhere before and you know the piston/cylinder isn't getting un-worn. Same, the noise fades away after a little driving, and unless I start driving just at startup (try not to), I can't hear it while under way. I've been meaning to pick up a compression tester, sounds like this is a good time.

Thanks for that. You say just piston slap. Does that mean you think it's generally not a big deal whereas rod knock would be?

Thanks for the replies. Tensioner (and belt, etc) was replaced 1300 miles ago. Doesn't mean it couldn't be bad now, but I'd be surprised. The belt felt and looked good when I pulled the side covers off last weekend but I'll pull the center cover this weekend to be sure. Through the stethoscope, the left block half is noisier than the right, but my lack of experience means I wouldn't say anything conclusively.

98 Legacy GT 5MT. 191K mi. on the car, now on the second motor, a JDM EJ25D with somewhere around 120K mi on it. Timing belt/idlers/tensioner/water pump and clutch/flywheel/pilot bearing/throwout bearing/clutch fork replaced 1300 miles ago. Oil is Rotella T 10w30. What I have is a rapping or knocking noise that’s engine speed dependent. It builds in volume after a cold start (cold engine, not freezing temps around here), with the noise starting 10 or 15 seconds after the car starts. You can hear it clearly sitting in the car, but it’s not obvious to me over other noise in the engine bay. Does not get louder with engine load, like I would expect big end knock to do. It’s not there (or not noticeable) when the car is up to temp, but the motor is generally noisier with more vibration than it used to have. Some of that is valvetrain noise, which is the most obvious in the engine bay along with injectors. I haven’t been able to get a decent recording of it or I would post that up. Does that sound like piston slap, rod knock, or something else? Head gasket on the left bank has gone, and I’m sorting out wether to rehab this motor, put in yet another motor, or move along to something different. Car is pretty clean and is rust free, but also needs suspension work, shocks and bushings all around. Any help is much appreciated.

Permatex green went in the trash can. That stuff was strangely sticky for something meant to promote sliding. I cleaned it all out and use the Valvoline high temp syn grease I had.