My 1995 V6 mustang makes a noise that sounds a lot like it's got an exhaust leak. The first time I had it checked out, it did have one, but since it started making the noise again, I had it checked again and now it's not. A lot of the V6 mustangs that I hear make the same sound. Does anyone know what this sound might be? A guy at a muffler shop told me it could be a small donut in the engine, but you can't just buy that donut, you have to replace the whole part and it's $600 from the dealer. Help.