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tapping noise between 2400-3000rpm

Wife  and I gave our 86  5000 cs non turbo to our dauther for her use.  We
bought the car with 15k and it served us well.  It presently has about
120k and has been kept after so that it still a pleasure to drive.  My onl
y complaint is a tapping sound which started many many years ago.  It
starts at approximately 24,000 rpm and stops at 30,000rpm, especially
under load conditions.  If I stomp hard on the foot brake and  gas the
engine until it hits the 24-30k range, it is loud as can be and it seems
to come under the car in the area of muffler located behind the cat
convertert.  If you also raise the hood and listen, it does not seem to be
coming from the head.  For many years I thought the tapping sound was
coming from the engine until a competent mechanic said it thought it was i
n the exhaust system.   Since then I have had the exhaust manifold  off
and machined,  cat replaced and both muffers replaced over the past 5
years.  The tapping sound has continued unabated even though it seems
somewhat louder these days.   
I have swapped cars with my dauther and put her car in the shop to once
and for identify the source of this noise and correct the problem.  My
mechanic who is quite good  spent considerable time trying to locate the
source but to no avail.  He thinks it might be the cat , but  it was
replaced 3 years ago and did not cure the problem.  He is also speculating
that it may be the belts pully causing a harmonic vibration.  The car has
not been wrecked.  I checked the compression and all cylinders were with
manual specs.  I do recall exactly when or how the noise  started many
years ago so for it was less audible way  back then.  I would like my
daughter to keep this car until she finishes her education.  But for this
noise, it is a treat to drive.   Has anyone experienced  a similar noise
in the same approximate rpm range.?  Does anyone have any sugggestion on
where else to look for a solution.    It may be my imagination but I think the tapping is not as pronouced
when driving in the rain, but it may that the wet road noise masks the
tapping sound somewhat.    I am ready to put some money into long term
maintenance items and would like to cure the tapping noise at this time.
HELP!      George K      j kot @pitt.edu