This one is from personal experience...

I wrote:
> Greetings list,
> I have developed a rather annoying vacuum related squealing/whistling
> sound... It is difficult to pinpoint, but seems to be coming from in back
> of the engine. I checked out as many of the vacuum connections as humanly
> possible, but haven't been able to track down the culprit. It seems to only
> happen after the engine is warmed up.

Check the bolt that attaches the throttle shield. Believe it or not, that bolt hole actually leads inside the intake. This bolt MUST be in place and tight!

If this is not the location, you can use a stethoscope or siphon hose to locate the source of the noise. With the engine running, press one end of the siphon hose to your ear and the other to the engine block. Then press it to another part of the engine block. If the noise gets louder, then you are getting closer to the leak.

> So, has this happened to anybody before?

Yes, it happened to me the day before I drove to DOA Nashville Expo, right after I bled my cooling system. I had not tightened the aforementioned bolt sufficiently. My DeLorean would run, but that squealing sound scared the bejeezus out of me!

- Mike Substelny
VIN 01280, 8 years

If you want to see the effects of having the bolt loose, check out this picture. Over time, the gasket slowly bent inward towards the intake, eventually becoming a reed instrument! On my car, the bolt head had broken off. It took 3 hours of torching, vice grips, and choice words to coax it out of the intake...