noise during deceleration

A year ago I rented a same model 2004 Sunfire for one month. Now I bought one this past August.
When I drive on the highway and have to stop quickly (like when coming up on stopped traffic) I hear a very fast load noise - a clunk or knock that sounds like something spinning then hitting something on each full rotation. I hear this everytime when in this situation. I have not noticed any shake or feel in the steering or brake pedal.
However, when I brake at city street speeds or coming to a gradual stop I do not always hear any noise. When I do hear it, the clunk sounds further apart between hits and is of less volume. The only pattern to the sound that I am beginning to notice is that I can hear it at slower speeds only after having driven on the highway.
As I never heard this noise once in the rental, I have taken the car both to the dealership and a GM service centre. They have checked the brakes and found nothing wrong. The car went through a full safety by the dealership when I bought it and had the tires aligned, and maintenance updated.
Has anyone heard of this situation or have any ideas about it?

Dude.. find out anything yet?.. iv having the EXACT same problem for the last month... HELP heh

thanks