H744 regulator whistle

I have two H744 regulators in my PDP-11/40, along with an H7441, H745, and H754. I also have two spare H744s and one each H745 and H754. All the H744s emit a fairly loud whistle, and it's driving me nuts. I've seen a note somewhere that a loud whistle is caused by the coil de-laminating, but I don't think that's the case. I tried swapping the two original H744s for spares, no difference. I also tried swapping the coil from the spare H745, which runs almost silently, and that made no difference either. I don't think I can just remove (short out) the coil either, as this is a switching regulator and the coil is needed to make it work properly. Any suggestions? -- Pete Peter Turnbull Network Manager University of York