How about a home made stethoscope? Just stick some soft rubber tubing a little way into your ear, and move the other end around. Admittedly this works better for air flow, eg balancing twin carburetors, but it might be worth a try.
And then there's the old way of telling if there's a train coming: use a metal rod to hear vibrations at the opposite end of the rod to your ear.
Let me know if my crazy schemes work?