5KCST Vacuum Noise

[Steve Sherman]

I've had what I believe to be a vacuum leak that is causing the usual
vent air problems on my 87 5KTW.  I've searched the usual culprits and
found/replaced a few worn/leaking hoses,  but still don't get vent air
50% of the time.

Recently a new clue started happening.  Upon starting the car there is a
"buzzing" from under the dash near the programmer box.  The sound is
like a reed vibrating quickly, almost like a buzzer.  It lasts maybe 3
secs or so when it happens.  And it only happens right at the first
start of the day, or after the car has been sitting for a few hours.

So far I have not been able to localize this.  I suspect it is a vacuum
leak causing the rubber hose to vibrate for a few secs until it settles
to some equalibrium state.

Since it is several hours to get the noise to repeat, and an awful lot
of hoses to look for a small tear in... I thought I'd ask the experts on
the list.  Any BTDTs out there that recognize this noise, and can give
me a clue where to look first?

TIA

Steve