Blue93

I am the original owner of a 93 SC400 with 150,000 miles. It has recently developed a humming noise when I lift my foot from the accelerator at 45-60 mph that is frequently accompanied by a slight thunk noise. The noise is not present on acceleration. The dealer has tried to sell me a new rear end. My local mechanic is convinced the rear end is fine, but does hear something in or around the transmission. The tires were just replaced and had no impact on the noise. Anybody have any ideas on what this noise is? Thanks.