(Sorry for this being so long, but I'm trying to tell everything so you
can try to make the whole picture of the problem)

I have a Audi A6 1996 with 2.6L V6-engine, with hydraulic valve lifters.
The car has been driven for about 85k miles (<140k km)

One and a half week ago when I started the engine, there was loud clatter,
like when the valves don't work ok, but this one was _loud_, believe me.

The oil change was due in near future, so I booked time from local
repairshop for oil and filter change.

I don't know what oil's there have been before, but when I bought the car,
I applied Mobil 1 0W-40 synthetic oil. I've only changed the oils once,
before this week.

The repairman said that the valve lifters need to be changed, as the noise
was too loud for normal operation.

So the valve lifters were changed, as well as the oil and filter.

And the noise didn't go away. Under the valve lifter gasket, there was lot
of ... something (not dirt, but something), so we figured out to try
engine flush and new oil change, if the something had clogged oil
journals (the oil channels are called journals, aren't they?) from other
V-branch. Also, when the gasket was removed, and the engine started, no
oil came there, just few drips from somewhere. So it seemed that the clog
in some journal stopped the oil from entering the valve lifters.

The flush seemed to do the trick, but when I had driven for few miles, the
clatter returned. Next day, I drove the car back. At first, the clatter
was there, but when I arrived at the repair facility, the engine was
quiet.

They did the engine flush again, along with the oil and filter change.
This time I drove around for 20 minutes nice and slowly, and engine seemed
ok. Just when I entered a freeway, the clatter returned. When I headed
back to the repair, the noise went away. I drove a little with higher rpm
(~3000 rpm), and the clatter returned, and when I drove more carefully, it
went away. For some moment, the clatter didn't return even with higher
rpm, but then with one faster acceleration, the clatter returned ok.

So it seemed when the rpm's were raised more than average, the clatter
came, and when driven slowly, it went away.

So what could be the problem?

I have been given thousands of hints, but not sure which one to try first.

One is the something in the engine, which might clog the journals again
and again. Several flushs might do the job, or maybe some other methods?

Can the over-pressure valve at the oil pump be defekt, so that it
starts to leak always when the rpm's are raised, thus negating the oil
pressure raise when accelerating? I suppose the oil pumps themselves
usually don't break down with this mileage, as well as they usually tend
to work better with high rpm's. The oil pressure light never lits, but can
I trust it?

Something wrong with the oil filter, or the interface between the engine
and the filter?

The rotating rod (shaft? cam shaft?) which moves the valve lifters, is/are
ok. What else should be checked before the engine is totally torn apart?

Hopefully someone has some ideas, any good advice would be appreciated.

 -Tero