2.8 V-6 valve tapping
Two years/12,000 miles ago my 1995 90S had head gaskets and timing belt/water pump replaced. Now with 148,000 ODO miles there is a severe valve tap. Recently had new oil check valves and valve followers (lifters) installed under the valve covers.
The tap is only heard after RPM remains above 3000 for several miles. Returning to idle the tap remains for a minute and then returns to normal, (silent). Engine load has no effect, same result in neutral as in 5th gear.
Oil pressure gauge in dash shows full (4.5 BAR) and once oil is hot will show 2.5 at idle.
Suspecting a blockage in the oil pick-up screen, the pan was removed and no material was found, the pan gasket was also replaced with the head gaskets 2 years ago.
With my local mechanic and the Audi dealer both puzzled I am now thinking the oil supply to the top end was effected with the head gasket work. The check valve/lifter repair was a waste of $1200.
The tapping began after the head gasket work but has become more severe with time. I hesitate to authorize pulling the heads again, I've spent $6000. in repairs at the Audi dealer in two years.
The other theory is a failure to supply oil volume but the clean pan and good gauge readings should negate this. Replacing an oil pump is expensive, ($600 part lives under the timing belt).