In BTTF1 at least, in the car park when you first see the DeLo, the sound of a Star Wars landspeeder is overlaid on the engine (thats the whine), which saddened me, cos I thought it was amazing.
The car gets suspiciously more throaty by BTTF3 in the "Hi-ho Silver" bit.. (but still with the Landspeeder whine), with the engine sound that the rest of you have mentioned.
And the door noise. Dammit, I really, really wish they made that noise.
:)
Matt