That sounds like a vacuum leak. Have you tried listenening with a
"stethescope" made of about 4 feet of garden hose? run the motor at idle and
put the hose VERY near the vacuum fittings.

OR

With an extinguisher nearby, with the motor running, spray carb cleaner or
(non-flammable-type) Liquid wrench on each of the fittings individually, and
seqentially. When you hit the one that is leaking, the pace of the motor will
change.

Best of luck, and happy motoring

GMB, Fairfax, VA
lookin' a '87-'90 Westy
1985 Jetta Turbo Diesel
1976 Transporter (AGAIN?!!)
ex 1976 Transporter
ex 1972 914 2.0
ex 1973 Squareback
ex 1985 GTi
ex 1970 914 1.7
(family) ex 1972 Transporter
(family) ex 1962 Microbus
(family) ex 1958 Karmann Ghia