you are going to trade in your car over a rattle?!? yikes, isnt that a bit over the top? you live in IL, probably rusted heat shield or a hunk of pothole tossed up onto a brace or bracket. I lived in Chicago for 27 years and being a mechanic, know how hard it can be to track down a rattle or squeak with all that rust and road debris, especially when its still cold out, everything seems to creak and pop and rattle when its F'n cold out. just give em a chance to find and fix the problem!
[I]"Winners never quit, quitters never win, those that never win or quit are idiots."[/I]