I own a 2000 Ford Mustang Convertable, automatic transmission. For awhile now, I have had this horrible sound coming from under my hood when I accelerate and pretty much whenever my foot is on the gas pedal. It sounds like I have a diesel engine. I've taken it to get fixed a few times, and they always say nothing is wrong, but the sound is getting progressively worse. I have tried adding higher octane gas and other additives and nothing helps. Does anyone know what could be causing this or ways to remedy it? Thanks for your help.