[Turd]

My 2001 Boxster S has over time developed a loud (from inside cockpit anyhow) howling noise that seems to be coming from the rear of both sides of the car. It really sounds like tire noise, (I have Michelin Pilot Sports) but the sound does not change on different asphalt surfaces. I have two new axles, replaced tranny fluid, rotated tires, tried my spare to determine if sound changed (sound did not change). Transmission works great. When I drained tranny fluid it looked okay. I put a magnet through fluid and have no filings. Maybe the Boxster rear end is supposed to be loud and I am just getting old and irritable! I guess next attempt would be to change rear wheel bearings. Note when I rotate the wheels while on jack stands with wheels off, both sides sound the same and don't imply the wheel bearings are suspect.
Minor detail......"... it's a 6 speed standard. Sound is same in all six gears and with clutch in.
Any ideas?