Weird Sound From Rear Driver Wheel

[chuanlee]

This probably has nothing to do with BMW at all, but I am getting a weird sound coming from my rear wheel on the driver side.

It's not the same as the occasional soft screeching sound I get when I brake or make turns. This new sound is almost like a wind instrument or vuvuzela, blowing a consistent note. It goes away when I'm going fast, but comes back when I'm going slow, and it's very loud; I am getting no light on my on board display, and no warnings on the GPS screen. Any suggestions what it might be? It's a 2009 535i if that matters at all in your analysis. Thanks in advance guys. Sorry if this is a noob question, but I've seen all sorts of answers in other threads I've found.