HELP: Abnormal Humming Sound increasing w/ speed

[cpiuma]

I have a 2000 Porsche Boxster S with 18k on it, "Pre-Owned Ceritified" with about 8000k since I picked it up from Porsche. Just recently I noticed that with increased speed, comes a lounder humming sound that appears to be comming from the engine compartment or the back of the car. This sound is seperate from an accleration engine noise and remains constant when you lay off the gas completely. It is especially noticeable with the top up & seems to almost contain this annoying higher-pitched humming. For example, when accelerating to 70mph on the freeway, the sound gets louder and louder...when rolling at this speed with no gas applied, it still makes this sound. It only occurs when moving, and the faster the more humming. Therefore, you cannot here this humming when you start the car or at rolling speeds. It is not unbearable, but definitely noticeable and abnormal.

The car has 17 months left on the warranty it isn't a problem to bring it in an get it serviced. I'm curious if anyone has any ideas as to what the problem might be before I go to those lengths. Thank you!