I might try and make my own version of the BTTF 3 Startup sound, might need help sourcing which sounds I should use. From what others have said above, It's a mashup of when it comes out of the truck, and when it gets started via the remote control. When I watch the scene I hear a diffrent/new sound and then the truck one half way through.
12 July Update: It's a Nightmare trying to do this, I'm trying to remove the Music from the scene but There is No balance between it sounding good and No Music. It's a lot tougher than I thought it would be but I've got an 'Ok Sounding' one, It's probably better creating it from scratch.