If I had to hazard a guess, I'd say it was capacitor-related. Large filter capacitors in the power supply circuitry can often make a whistling noise when they start to go bad. I've had a few that have done it in peecees, though I never tried to actually replace capacitors; I just replace the PSU.
However, since we're talking antique Macs, I'd be more likely to give shooting and replacing them a go. It would be a bit of a craps shoot as to which ones were bad just on the whistling alone, unless there was obvious damage or leakage, or someone used an oscope on them. The only oscope I've got is a tube-based model from the 50s, and I don't think it works anymore, so I'd just replace 'em all, cuz I'm crazy like that.
