Exterior bumping noise at all speeds

my Police Interceptor has 82k on it. Rather severe winter Goodyear tires on rear. At highway speeds, and above 45 mph, I hear a noise similar to the flap of helocopter blades..best description I can give. Increases with speed. I wonder if it tires or soemthing else. I don't feel any tire thump and tires are round..jacked up rear and spun wheels by hand with tin can next to tire...aw, c'mon..you've done the same thing over the years. Harold in Iowa my Police Interceptor has 82k on it. Rather severe winter Goodyear tires on rear. At highway speeds, and above 45 mph, I hear a noise similar to the flap of helocopter blades..best description I can give. Increases with speed. I wonder if it tires or soemthing else. I don't feel any tire thump and tires are round..jacked up rear and spun wheels by hand with tin can next to tire...aw, c'mon..you've done the same thing over the years. Harold in Iowa