Strange noise from GT40

I recently purchased a 97 SNOB from a local marina. It was a trade in on a new one. When I test drove it in October, I knew I wanted it. Only problem was, 20 minutes or so into the test, it started making a loud humming noise that could be heard over the engine at about 2700 rpms. It got louder and quiter as the rpms raised and lowered. We shut it off, waited 2 or 3 minutes and restarted it, and it never did it again. The owner of the marina was with and he said that it was the belts humming and they would replace them over the winter. Anyway, long story longer, they changed the belts, oil, and impeller, but the boat did the exact same thing the first time I took it out last weekend. It will run for hours at 1900 or so RPMs, but once you kick it up for any extended amount of time, that noise starts again. It's got the fuel injected GT 40. Any ideas?

Thanks