'99 A4 2.8 engine knock

Hello -

I have an annoying engine knock on my 99 2.8 A4 (145k mi) that's puzzled the
mechanics that haved attempted to diagnose it. Here are the symptoms:

-When the engine gets warm, there is a knock/rattle noise coming from the
lower part of the engine on the passenger side.
-The hotter the engine gets, the worse the knock gets.
-The knock can only be heard when the car is idling. When the engine is
revved over 1000 or so, the engine sounds perfectly smooth
-The car's idle is somewhat rough
-The knock persists when high grade gas is used (only a handful of times
have I used less than premium since I've owned the vehicle)


What I've been told:
-Timing belt. *I am somewhat skeptical of this, as I had the timing belt
replaced at 90k (55k miles ago).
-Oil pump chain stretched and hitting side of engine. *Not sure about
this.

Has anyone experienced anything similar or have any thoughts? Any input
would be VERY much appreciated.

Thanks!

--Ted