The video link below is on my 1999 Outback 2.5L DOHC motor that I just got done replacing the main and rod bearings on. The pistons had some slight wearing on the skirts. Motor had 200,000 miles. I replaced the piston rings as well.
You can hear the noise in the video. It seems to vary with temperature. When dead cold, it sounds like dieseling and it is louder. When warm, you don't hear the noise at RPMs above 1,500. You don't hear it while under load when warm. While it will change with how warm the engine is, the noise is always there. It never completely goes away.
I've changed out the timing belt tensioner and the noise is uneffected. The noise is definitely on the driver's side. I've listened with a mechanic's stethescope and my best guess is that it is either the front driver piston or the valves front valves. It is possible that the waterpump could be the issue as it is right down there in that area and it was stamped as remanufactured.
So, I'm looking for thoughts and suggestions. I really don't want to have to pull the motor again, but you got to do what you got to do. I've not moved spark plug wires around yet and I've not cleaned the MAF. My experience is that a dirty MAF will exhibit knocking while under load, not at idle. Besides, this sounds more like a mechanical issue.