You may have a rear shock or strut malfunction.
〉 Answered on Sep 8th, 2009 by Amanda J. Pierce, MBA "AJ", Certified Mechanic, Project Coordinator; Primavera Scheduler at VIGOR Alaska Industrial Shipyard & BizzM3ch Solutions
First, I would suggest that you look it over really well. You may be able to see something stuck in the tire. Next, check the air pressure. Some tires do make different noises if they are not inflated properly. If you still have not found the cause of the noise, I would suggest bringing the vehicle in for a quick inspection. You may have a flat spot on the tire or something stuck in the tire that you cannot see.
〉 Answered on Sep 8th, 2009 by Suzanne Grego, Technician at City of Philadelphia Fleet Management