2001 Boxster S howling noise

[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?

[gregwils]

Noises are always hard to identify through a forum. How many miles on the Pilots? If the tires are starting to feather even slightly, they can really be loud regardless of surface. If you can take your car someplace that you can put it on a lift to run the motor and trans through the gears, it may help you determine if it something other than the tires. Or, if you know someone with a Boxster that would be willing to swap rear tires for a quick test ride.

You are already did what i would have done, only option would be to continue process of elimination.

[Sharkbit]

It may be coming from the gas fill area. There are two valves in the evap/charcoal cannister system that can go bad. They went out in my Boxster and it sounded like a howling/trumpet sound, came and went, RPM did not matter, and one time even did it at idle. If you look at a diagram of the gas fill/evap area you will see where they are located. Not are hard fix. Just a pain to get to. Hope this helps.
http://dcauto.gotdns.com/illustration/index/2701906