My 03 sedan (150k miles) has this really odd knocking noise coming from the engine bay, and appears to be coming from the driver's side valve cover. I'm positive this noise isn't rod knock, but it seems to be some sort of valve train noise. The noise becomes more audible when I remove the oil fill cap.

The noise isn't loud at all, but I can hear it when I'm in the car at idle, in fact it almost reminds me of the sound of an old tractor (if that makes any sense?) . I'm so OCD about my car and this is driving me crazy!
Anybody know what this might be?

 

Sounds like a lifter tick. I have the same problem

 

It's more of a knocking diesel noise than a tick. I know it isn't the rod bearings, and I'm almost positive it isn't anything to do with the pistons or crank. I've been doing some research and it appears to actually be the fuel dampers. I used a stethoscope to listen to them and that seems to be where the noise is coming from.