2.4 NA, 200,000 miles.

I'm getting a squealing noise that sounds exactly like a slipping fan belt, however a new serp belt didn't cure it. Here are the clues:

1.) It only happens after driving for a while and the engine is good and warm.

2.) It's somehow related to crankcase vacuum. When the squealing is happening, the oil filler cap is very difficult to remove. It's REALLY sucked down tight. Once the cap is lifted up, the squealing stops immediately. Screwing the cap back on, the squealing restarts immediately.

3.) The noise seems to emit from somewhere below the #4-#5 intake manifold runners. I have put the stethoscope on everything I can find in that vicinity and can't hit on the source of the noise.

4.) Turn the A/C off and the squealing SLOWLY diminishes. Turn the A/C back on and the squealing SLOWLY returns. (My theory on this part of the puzzle is that that when the A/C is shut off, the reduced engine load translates to a change in idle control which affects the vacuum.)

I've read that there is a vacuum pump that can act up and cause issues something like this. Also, this car is one of those that will occasionally emit a short (1-2 seconds) "buzzing" noise every few seconds after the engine is shut off. Is this related?

Any ideas what might be going on?