For the last month I have been hearing a "revving" noise from the engine...but it's not the motor rpm's and not a speed increase. At a normal 30-50 mph it will sound normal and then with no acceleration, I hear the revving, turbo boost gauge does not change and rpm's does not change. The revving sound may go on for a few seconds or up to 15 seconds and then stop? Ford says they can't detect anything wrong when it was in the shop. The more I read here I am thinking its the fan cutting in and out? It didn't do this when it was hot (summer)...why start now? Any ideas?