Sorry about your luck, mate, and let's hope it's just the hydraulic lifters "pumping up."
My independent Audi shop has published this as "usual" for Audi engines with +100k mi or so, and that it does not present a problem.
A LOT of years ago, I listened to valves by holding a stick against the valve cover and putting my ear up to the end. There was a wonderfully different sound, and the solid lifters made a sewing-machine like clicking, quite mechanincal. I suppose now you could use a stethoscope...
Anyway, is it possible for you to poke under the hood (bonnet?) when the car is started, and isolate the sounds to either the valve train, or the vacuum pump area? This may help the diagnosis.
Best of luck.
'96 E300D 58k mi (wife's daily ride)
'95 Audi 90 118k mi
'92 GMC Suburban 138k mi
'85 300SD 230k mi (my daily ride)