Weird Noise From Rear

97 Lumina police package with a noise from the rear thats kinda hard to explain. It makes a "whoop, whoop, whoop" sound, when I am braking OR just rolling. I was trying to figure it out at a real slow speed (about 2 MPH) it almost sounds like something flexing, not scraping or grinding. Brakes are fine. Anyone else with a similar problem? Any ideas? Thanks

Abnormal tire wear/broken steel belt will produce some really strange road noises in the rear...it's gonna be that or the hub bearing, most likely...

Jeff, I'm going to lean more toward the hub bearings. The tires I have are almost brand new, and I also swapped them around and the noise DID NOT follow the tire swap. Thanks!