Engine 'roar'

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.

If this is not something you want to attempt on your own, have someone at a tire and alignment shop go with you on a test-drive so you can point out the noise. That way you are sure they are looking for the right thing. They can usually figure out where to look just by listening to the sounds, then they can narrow it down with a stethoscope while running the car, in gear, on a hoist.