A few months ago.... I posted this. It may help.
Originally posted by Tony2005:
..If it is an exhaust strap or shield loose, you could check by having someone rev it in park while you look below (don't go under the car). On the other hand, you may want to try taping or tying a cheap camcorder under and recording the undersides while driving. I have a $100 one from Target (which records for 5 minutes or so).
Don't use the $1,000 ones though (the person you borrowed it from might not appreciate it). 
