On my wife's '02 boxter there is a high pitches squealing noise similar to something rubbing on a brake rotor that comes on at about 45 to 50 mph and doesn't go away until speed drops to below 20 mph. As I slow down the frequency of the sound goes lower as well. Since the brakes needed replacing I replaced rotors, pads and sensors. Nothing abnormal was found during the service.

The sound doesn't seem affected by cooling fans, brake application, sharp turns or air conditioning being on. I took the car to the Porsche dealer and they can't find anything. I went under the car and searched for any loose panels, cleaned the radiator grilles for anything loose, checked for backing plates rubbing on the rotors and anything else of a subtle or obvious nature.

So far nothing has been found and it's driving me buggy! Anyone have any suggestions? I also noticed that on the back of the hub shaft (front wheel) there's a serrated wheel. Does this have anything to do with a speed sensor? If so Is it possible that the sensor might be vibrating or loose?

 

Wish I could help. Seems I have the same problem.. Seems to be in the rear. Had car checked out and not brakes. If you find out please post answer. thank you.

 

Squealing problem, update: I took the car to a shop that had a master mechanic (not a Porsche dealer) and he removed the left front wheel bearing and said that it was the problem. Since it's a sealed bearing, I couldn't tell exactly what it was but the reason it didn't show up when we felt the hub by hand was because it wasn't under load. That's where the problem showed up. So the problem is gone and I can get on to other, more important things.

 

Yup, wheel bearings are pretty common problem. Since it is a sealed bearing, you won't have play in the wheel like a traditional bearing that is loose or shot, but instead you will hear a growling at speed, similar to how tires sound when they are cupped.