vacuum whistle at cruise

(85 300SD, 294k)
Vacuum whistle coming from vacuum line that goes into the turbo. Some sort of bypas diaphram. Only "whistles" at cruise. Accel or Decel = no whistle. Very loud at 60+ mph. I've stopped it by pluging that line. I've looked for a rebuild kit or a new diaphram but can't find one. Only started this after about 200k or so. I've been running with that line plugged ever since. Doesn't seem to effect performance that I can tell.

I've just found this site. (I go to one similar for my 96 GMC 6.5 diesel.) Has anyone had a similar situation? I'd rather get this fixed, but no one can find a part number for a replacement diaphram, (other than replace or rebuild the whole turbo).

Thanks...