If it happens when the car is not moving you can use a mechanics stethiscope to listen around for it. Get a long screwdriver or prybar and place the poinst of the tool where you think the sound is coming from. Then just put your ear on the plastic part and listen. Keep moving the thing around till you figure out where the sound is coming from.
Be very, very, very careful while doing this though. You are going to be putting pressure onto a unstable object with your head so make sure you are holding it firmly with both hands. If you have lone hair like me make sure you tie it up and out of the way.
1989 LE Bonne 122,000 miles
Custom Open Element filter system, Strut Tower Brace from 2nd gen bonne
Light Up Pontiac symbol in grill, 16 wheels from newer Bonne/G, performance tires, performance ignition system, Upgraded brakes.