Knocking/ticking from head, please help!
 

A couple weeks ago on my way home i started getting a loud knocking coming from the engine (99 boxster 2.5) (way louder then i would describe a tick). It was very quiet at idle (but still present) and it increased with RPM with a big increase in intensity after about 2000 rpm, it did not change whether the car was cold or warm, and there did not seem to be a loss in performance. But it was very loud, definitely not something to ignore.

I lifted the car up and used a mechanics stethoscope and it seems to be coming from near the forward most exhaust valve on the drivers side. I took the car into the shop and they confirmed the same thing. The shop did point out that they did NOT hear any abnormal chain noise so they also believed it to be valve related.

I pulled out the cams and tappets so i could visually inspect everything, as loud as the noise was i was expecting to find something quite obviously wrong. but i can find no damage or abnormal wear in the valve train at all. Everything visually appears to be in order.

I am kind of at a loss as to where to go next. Any suggestions would be greatly appreciated!

My best guess is to check for a fully or partially blocked oil passage that feeds the valve lifter (aka tappet) or within the lifter itself. Little or no oil means that the valve lash may not be fully taken up resulting in valve ticking.

I did a more detailed examination of the lifters. I noticed that lifters that would be closest to the front of the car did not go down as far as the other, i have no idea if its supposed to be this way or not? Does anyone know??

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After that i noticed that since they dont go down as far, the bottom of the lifter is gouging the carrier that they sit in...

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I may i think i found the culprit making the sound. The problem is that i dont know WHY its doing that. The lifters are VERY firm. The valve and valve spring appear to be in perfect shape. Am i way off base here, or do you think this is the issue?? And what might be causing it?

Any ideas??