I have a 1968 Mustang with a 302 V8 and a C4 tranny. Sometime at idle and at shut off I hear a low bombing sound. Almost like a base drum, it starts and then goes away. No sound as I am driving or when I gas it. I do have power steering, but it sounds of a tranny or engine origin. By the time I get out of the car and open the hood it's gone. Help!