I'm having the same problem. In addition, the radio signal is interrupted by a quick burst of static every minute or so (I haven't timed it exactly). The output from the media player is not affected, and sounds great through my powered speakers.

I normally leave Bluetooth turned off, and FM Radio turns on the module for me. Could this be the problem? Is the application somehow mishandling turning the BT module off and on again? I find that the application works if I just leave the N900 alone for a while, but not if I turn the radio off and back on in a few minutes. Again, I don't have specific times, but can try to measure if needed.