While not quite as bad as some of the more recent submissions, here is my winner of the day.
2002 Ford Taurus
Customer complaint: Constant grinding noise while driving, burning smell. Check L/F tire losing air.
Suspected symptoms: Worn brakes, bead leak or nail.
Here's what I found.
A broken coil spring, wore through the back of the tire.
I almost had one better than this, but the customer didn't want to take it apart. 2005 Nissan Titan. Growling noise while decelerating... The rear diff pinion had 3/4" or more of in/out play.