We seemed to be having this problem as well on our older TT, but we took off the filter and noticed that the fan blades where hitting the little 'rain flap' that had somehow gotten past a little lip that kept it away from the fan blades. I just reached in there and pushed it out - it happens everytime that we move the trailer (I think it is the wind coming in from the outside as we are towing). Might help, might not, depends on where it is hitting. I would take off the little metal filter and see where the noise was coming from.