Something loose?

Talked with Ken today and he thinks there must be something loose that is causing a vibration noise. He's been running the engine some more and finds that it goes away when running a speeds higher than idle, but makes the noise when deccelerating (backing off). That suggests a loose flywheel, clutch plate or harmonic balancer. Guess we need to use a stethescope and listen more closely to see if we can pick up the source.