If, when you turn the wheel and the noise increases, you are not moving, then I am clueless too. However, if you are in motion, the noise is from the fan. The bushings on the fan shaft are a "self lubricating" type. However, they dont last forever. They are of a material like an oilite or something similar (lubricant-impregnated brass). They have to take care of not only radial, but also axial loads. When you turn a corner, the axial load increases, making a loud bushing louder. The worst example of this I have seen (excluding my own W201) was my old Maserati. The fan stopped working periodically and would restart if I hit railway tracks when turning right. I used to go out to a place I knew where I could determine if I could get a few more days out of that fan, or whether it had finally siezed altogether. You might get a few more months out of your fan, like I did, by taking the cover off and using a pinpoit oiler to put a little 3 in 1 on the shaft and bushings. In the summer, however, this will attract dirt and wear the bad bushings even worse, but you can at least prove the fan is indeed the culprit. Don't forget the obvious of turning the ventilation system off completely to see if the noise goes away first.