Yeah, it hit me that we were probably talking past each other, i.e. hydraulic lifter=lash adjuster
So, let's get back to what you've done so far...you've used a stethescope and checked around on the engine and haven't "heard" the sound, so I'm going to say that it's not injector (you have put the 'scope on the top of each injector, right?) or lash adjuster noise (you have put the 'scope on each valve cover, right?), as you'd have heard it through the stethescope. You've checked the flexplate bolts, so it's not a loose flexplate. The noise does change with engine rpm, "going away" at higher rpms.
Suggestion - get the car either on a lift or up on jack stands and go back to work with your stethescope. If the sould is audible to the "naked ear", you should be able to pin point it with the stethescope.
The good thing is is that there aren't that many things on the EG33 that will make this type of noise. The bad thing is that they're all a PITA to fix
-Bill
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