Last week my '02 GC started making a strange warbling. whistling noise. After a few days I noticed that it didn't start until 25-30 mph and changed pitch as speed increased. It wasn't related to engine rpm. Applying brakes, turning steering wheel, and shifting from 2WD to 4WD had no effect. It seemed to be solely related to ground speed.
So I resigned myself to having to take it in for service, thinking it was probably something in the axles or driveshafts, meaning $$$!
Before doing that, I thought I'd crawl underneath and see if I could see anything obviously wrong. Everything looked dry and seemed tight. As I got toward the rear, I was checking the muffler which I know is starting to rot and needs to be replaced. As I reached up above it to see how bad the top was I brushed against the aluminum heat shield which started waving up and down!
The back end of it was hanging halfway down between the body and the muffler. A closer look showed that the rear attachment point had pulled through the aluminum. The shield itself was ok, it just fatigued or corroded around the washer and separated. Took off the speed nut, puy on a large washer, replaced the nut and all was tight again. A quick road test confirmed the noise was gone. Saved myself a $70 service fee.
Apparently the shield was either pinging against the tailpipe or the wind was whistling through the hole in the shield as the wind reached enough speed to make the noise.
Just thought I'd share this.