Lifter noise

[Richard J Lebens <rick-l@rocketmail.com>]

My 1991 90 Quattro 20 valve has started to make an annoying clack,
clack, clack.  There are 99,989 miles on the odometer.  I remember
various threads over the past years about lifters but I ignored them
figuring it wouldn’t apply to me since I religiously change the oil at
3000 and use Audi filters. 
 
But anyway what do I do now.  Would any kind of a chemical flush do
any good?  

Should I replace all 20 lifters?  Since the intake manifold has to
come off to get to the valve covers, how can you run the engine to
find the bad single lifter?  I remember some other audifan replacing
the lifters and the noise coming back shortly.  What else could it be?
 Anything else to look for.

I imagine the timing belt has to come off for this.  Can the water
pump be repositioned without draining the coolant or do you have to
take it completely apart and put a new o ring in?  It is only a year
old.  

If I decide to live with the questions about what is wrong with your
car, does this do any damage.  The car seems to run fine.

Thanks in advance for help with all my questions.
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