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Vacum Leak Causes A Chirping Sound (88 90q)

For the past two weeks the "fault light" has come on sporadically.  After
pulling codes I got Oxygen sensor and OXS Control.  I replaced the OXS and
knock sensor about 6 months ago, they should be fine.  Greg at M&M Audi Haus
told me it might be a vacum leak of some sort, causing the OXS control to be
out of range and causing the fuel mixture to be max rich (my exhaust pipes
are very black inisde).  I found that the lower ISV hose had a tear in th
bottom, so i repaced that easily and figure my problem was fixed.  Sure
enough, the light comes on still, just not as often as before.  After work
last night I reved the engine and listened for any sound of compressed air
leaking.  As i would rev the car using the butterfly i heard a quite
chirping sound, right when i gave it some gas.  I did it many times until i
pinpointed the sound coming from underneath the intake manifold.  My
question is, has anyone had any experience like this or know what it may be?
I really don't feel like pulling off the intake if there isn't a problem
there.  Could it by the gasket in the intake manifold?  Any help or advice
would be appreciated.

Mike Robinson
1988 Audi 90 Quattro
194,000 miles
San Jose, CA