Noise somewhere in rear of car. on 1991 Pontiac Grand Am

While at a stop sign, I heard a noise. Thinking noise was some kind of emergency vehicle siren emitted by the vehicle behind me, I pulled over into a driveway. Realizing the noise was from my own car, I got out to try to locate the source. Outside the car, the noise was not as loud. It seemed to come from the rear area and much easier to hear from inside the car. The noise seemed to last about two seconds, stop for one second. After about one minute the noise stopped. This noise first occurred returning home from a road trip 7/4/10 and again the next day at a stop sign. On the short time to the store etc. it did not reoccur.

Asked by for the 1991 Pontiac Grand Am
Best to have a shop listen to this noise, because there are so many things that can cause a noise like this, it is always better to deal with it in person.