Knocking/ticking from head, please help!

[neon]

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!