I think what you hear is not the fan failing but it has something to do with the current draw on the inverter. If I turn a few lights on the fan doesn't come on, if I turn a couple of more it sounds like the fan is trying to to start and stop. Add a few more lights and the fan speeds up. I figured the fan was going south so I replaced it and it does the same thing so it is not the fan. Been trying to find a schematic to see if there is a heat sensor on the heat sink which could be the problem, but for now I just crank on a few more lights.