Unidentified Noise

My 1992 Buck Century sometimes makes a whirring sound when I reach a speed of 45MPH or more. Sometimes it sounds steady or sometimes rythmic like "whir, whir, whir or whoop, whoop, whoop. It dosen’t do it while standing still, so I know it’s not the engine. It’s strickly a motion noise. Before the car refrains from making that noise for my mechanic’s ears, can anyone provide a clue for the cause? Is it the brakes or wheel bearings? Transmission problem?

Given the age of the car, have the tires checked for unusual wear and have the suspension checked for loose bushings, bearings, ball joints, and tie rod ends.

Thank you very much for your prompt reply.