Strange noise during acceleration

[beckwithguy]

My 77 450SL makes a loud high pitched noise during acceleration that sounds like a whistle or squeal. It is intermittent and the pitch changes or disappears when I back off a bit on the accelerator. It also seems to change pitch as the tranny goes through its shifts. It is more noticeable at low speeds and sometimes occurs at random. In those cases, if I touch the accelerator and increase the rpms it increases in pitch but then disappears. There is no noise when the car is in park and I rev the engine.

I've checked the belts and they seem ok. Am wondering if it might be a pump bearing or tied to the water pump. Am totally stumped by this so would welcome any suggestions.

Thanks

Mike