new lifters - still noisy

[Nick Craft <ncc@scr.siemens.com>]

Fellow Q-heads,

Sorry to bring up this perennial topic yet again.
My '88 5KCSTQ has always made the famous audi valve lifter "clicking" noise,
especially when cold. Mystery oil fixed it briefly, but it soon came back.

Last week I replaced all the lifters, expecting to solve the problem
once and for all.

Guess what? No darned difference. What is going on here. Can anyone theorize
what else may be causing this?
The clicks are at a frequency of about half the rpm's (e.g. 500 clicks per
minute at 1000rpm). Engine cold: always present < 4000rpm. Engine warm: barely
audible at idle, louder the more you open the throttle, but quiet when cruising.
In other words, loudness seems to be proportional to throttle  opening, not to
rpm's or to load.

Theories:
1) When you remove each lifter, you see a little oil squirter thingy. Could they
   be blocked? Can you unblock them by poking something into them?

2) Is something possibly wrong with a valve itself? or a piston even? or the cam?

3) What's this vacuum pump thing people keep talking about? Does my car even have one?

4) I use Castrol syntec 5W50 (?). Will a switch to something else help? Back to dead
   dinasours even?

I'd really like some feedback on these possibilities, and some imaginative new theories
from you lot. I'm determined to get to the bottom of this...

Cheers,
Nick "Sherlock" Craft.