I MAY have found the problem. There is a support for the exhaust that is bolted to the rear of the transmission mount. Mine was incomplete--I just had the arm/ cradle that holds the exhaust pipe--but no clamp. I fitted a clamp today, and will test drive tomorrow. The pipe appears to have been laying in the cradle, but with no clamp to hold it tightly in place it was able to move and buzz-- at least thats what I think now. I'll post back after I drive tomorrow.