Alright, so when I get up to around 40 mph and then let the car coast I feel it shift (I have an automatic) and then if I step on the pedal to accelerate at all while in this gear a strange noise will come from underneath the car from around the pedal area when I step on the pedal. It can be heard inside and outside the car, the loudest being outside. I can't explain the noise, but my girlfriend said it sounds a diesel engine, although the sound isn't coming from the engine, but it has that kind of diesel tone to it. Everything works normal, it just makes that sound while in that gear, anybody have any idea on what it could be before I bring it to the dealership?

 

If it sounds like a diesel it's probably detonating. Try running a higher octane fuel in it for a few tanks and see if it does the same thing. I know when I ran regular in mine if I really got down on it, it would detonate which can sound like a loose exhaust pipe or the familiar chatter of a diesel.

 

Thanks for the reply, after reading up on detonation it sounds like it could be the culprit. I found another forum that mentioned there was a service advisory on 2002 Mustangs for spark knock, so it's definately worth a trip to the dealership. I use regular fuel most of the time, so I'll have to switch. I had to get a new engine last January because one of the cylindar holes got mis-shapen, I'm wondering now if the detonation could have had something to do with what caused it. I just hope that this is what it is so I don't run into another dead end like all the other problems I have with the car. Thanks!