[Northkeep] Question for the mechanically inclined.

[kevinkeary at aol.com]

I (and the finance company) own a late-model Chevy Trailblazer, which 
is sort of like a preppy Jeep. It has settings for two wheel drive and 
others I've maybe used once for automatic four-wheel, four-wheel, and 
low-range four-wheel drive. This morning it developed a vibrating 
noise, with symptoms I've never encountered before.

The noise comes up through the floor in the back, where the tank (and 
presumably the fuel pump) and the exhaust system (the muffler but the 
catalytic converter should be farther forward, unless acoustics are 
doing something funny) and the rear differential are. But it only 
happens at fairly high speeds, over twenty miles per hour maybe, and 
only when there is no pressure on the accellerator.

It doesn't happen when the cruise control is on, presumably because it 
always keeps some positive pressure on the drive system. It doesn't 
shake the car or affect its handling or power. The lightest feather 
touch on the accelerator stops it, but it starts again 1 to three 
seconds after I let off the gas completely.

I would think if it were a blocked catalytic converter, it would be 
worse when accelerating, and the same if it were the fuel pump. It 
doesn't seem to fit a wheel bearing or a brake problem either. I 
worried about a U-joint on the driveshaft, but I found very little play 
in the system when I shifted from reverse to drive and back (while 
holding the brake with the truck motionless), and no noticeable 'clunk' 
sound.

Any ideas? Keigan is driving the vehicle, so if it IS a U-joint going 
out, I want to know that and get it fixed.

Kevin