u/Paumanok

Best description in title. Video didn't do it much justice. With your head around the brake booster, with the car idling, it's fairly loud and high pitched. If you go around to the passenger side, you can barely hear it.

Noise stops under throttle

Brakes about as good as they ever have, no extra noises under braking.

Posting here hoping one of the knowledgeable foxbody greybeards have any ideas.

I've been trying to sort out various vacuum line gremlins but I can't seem to find a decent 92 2.3 vacuum diagram.