A couple times now the AC fan started to 'sound strange' and did not blow at anywhere near its normal velocity. In both cases, I removed the grill over the intake, with no change in sound or operation. The first time, the filter was dirty and I cleaned it; the second time it was still clean. In both cases turning the AC off and leaving it off for a few hours before restarting restored normal operation. Anybody have any idea what is going on and how to fix it?