I will second what Kenny wrote about his "shrill" C turbo. It hurt my ears and he was still a mile away climbing the mountain to the '07 GA Rally. My C turbo is tolerable in the truck, but can be heard for 100's of yards away when the truck is at idle.
The D model can be heard when you shift gears. Here is a video Colton took of me driving Kwai's D turbo'd M109 onto I-10 at Ft. Stockton after we picked up the truck in Las Cruses:
[media]http://www.youtube.com/watch?v=5SPnXv4Vdwg[/media]
You can hear the whine just a little between 4th and 5th.
Pistolnut bought a C turbo truck that had been sitting for years. It idled and moved around, but never out of 1st gear while we got it ready to drive home. We never heard the turbo and not knowing then the physical differences, just figured it was a D. This was in the fall of 2005. I was following him as he started home. About 5 miles into the drive which he had not been able to get above about 30 mph, I saw a puff of smoke not related to gear shifting and through the rolled up windows of his Ford truck, the whine started to wind up. The hair on my arms did too. He was able to grab 5th, reach 56 mph and that thing whistled the rest of the way home. So, yes they can be stuck and even though they don't put out much boost, a non working turbo plugs things up so not much speed can be achieved.