runbling

2005 COLORADA 4x4 AUTO, 65,000. MY COLORADO JUST STARTED RUMBLING/VIBRATING AROUND 45 MPH. BEING AN EX bRIDGESTONE EMPLOYEE I HAVE RULED OUT ANY TIRE PROBLEMS. THIS PROBLEM SEEMS TO GET WORSE UNDER LOAD OR TORQUE , BUT ONLY IS NOTICEABLE AT 40MPH AND UP. IF I BACK OFF THE ACCELERATOR, PROBLEM IS STILL NOTICEABLE, BUT LESS RUMBLING. I BABY THIS VEHICLE. ONLY USED FOR PERSONAL USE, OIL, REAR END, TRANSMISSION CHANGED TO EXCESSIVENESS. THIS IS REALLY BUGGING ME, ANY SUGGESTIONS?

sounds like that problem with the valve seats.

does it have a rough idle?