I've had my E60 M5 (2006) for about a month now. A couple days ago I was driving it in the rain---first time since I've owned the car. While driving I hit the windshiled wipers and made a nasty sound, you know when you use the wipers and theres no water? It made that exact sound. I just took it as the wipers needing more water but that was not the case. It was pouring and the wipers continued to make that nasty sound. Any suggestions as to what might be causing this?