I bought a motor fan for the Mamba Monster off Ebay a while back, and it was doing good until about few months ago. When I flick the switch to turn everything on, it makes a revving noise. It's like wooo wooo wooo then it gets slower and slower wooooo woooooooo woooooooooo then normal.... After all the owl sounds for about 20 secs it returns to normal fanning. Any ideas? I checked for fraid wires but I don't see any.