Noise when backing up my car

Tie yourself to the bottom of the car with duct tape, then have a helper drive around while you listen with a stethoscope. Watch out for pot holes!

OR, a better idea might be to raise the car off the ground, either on a hoist, or jack up one wheel at a time, then spin each wheel by hand to see if you can make the nose occur.

There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.