1992 Audi 80 8C B4 2.0E Oil Spec

[esseff]

Here's one for the Audi gurus!
I've got a 92 8C B4 2.0E with less than 80,000 miles on the clock and have noticed a strange engine noise for the last couple of years. When I start up everything sounds fine but once the engine's hot after 30 minutes or so, there's a grating/clattering noise at idle from somewhere in the upper cam belt cover area. If I increase the revs a little, the noise goes away.
I couldn't pin down the exact source with a stethoscope so I took it to a local mechanic who said it was the cam belt tensioner pulley. It wasn't, so that was twenty quid wasted. Then he said it must be the alternator bearings. The price of having it overhauled wasn't much less than a new replacement so I fitted another. That didn't #*%%$@ work either so more money lost.
I'm wondering if it's a reduced oil pressure issue as there's no abnormal noise when the engine's cold. It's presently on 10-40 semi-synth.
The oil was changed about 4 or 5000 miles ago so maybe it wouldn't hurt to change it anyway as most of my trips are less than 12 miles. Any opinions on changing to a 20/50 grade for diagnostic purposes? Or any other suggestions?