I had this weird thunking noise inside my car. If you hit the brakes it would roll forward then thud, if you hit the gas it would roll back and then thud. I pulled of the int. door panel off nothing in there, looked everywhere else that I could think of like under the carpet....

Well I finally took it to the dealership and they found a tube of sunblock in the frame rail (or channel something like that anyhow). I guess if you have a mustang with factory power seats they just put an elongated cut in the carpet under the seats for the wire harness to travel (so you can put the seats forward and backward) and it leads to the frame rail where the wire harness runs through, and Somehow this tube of sunblock managed to finagle its way in there. So if anyone else has any weird sound they just can't figure out and it seems to be next to the passenger seat then in the back seat and everywhere in between this may be your problem.