'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.

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