Thanks for the help thus far. I did a check for motor mounts by power braking and checking for engine movement, no movement. Crawled underneath and tried to wiggle and move the exhaust, nothing as everything feels tight. Transmission dip tube feels snug.
Here is some more information that may help. About 3 years ago the cadillac converter was replaced as it coded to be plugged, weeks later I removed the heat shield because it was rattling. At times after start up and until things warm up I will get a rattling thumping sound that appears to be coming from underneath. It sounds like heavy metal on metal then goes away usually in about 5 minutes, but only noticeable when moving less then 20 MPH and goes away when idling and not moving. It could be happening at higher speeds but unsure. I can't reproduce this as it is very intermittent. RPM at 65 mph is about 1900 and MPG is 15.5 the last two fill ups. Above 65 mph the motor will begin to roar at a different pitch, but if gas is released it goes away. Then when gas is applied again above 65 it reappears. Unsure if any vibration is occurring, if it is it is very little to know for sure. Tire wear is about 50%.
Last edited by jekokmey; 03-16-2014 at 07:34 PM.
Reason: more info