I had a 75W power inverter that made no noise when being used, and I bought 150W that makes a humming noise. Any ideas if the humming noise is a bad thing, a signal that it's too strong? Thank you in advance. Read some threads before posting.
From the owner's manual, If you put a full load on that 150W inverter, I'd expect it to blow the fuse feeding the 12V outlet. I don't think that the inverter should hum, though, just because you connect it to the car.
When I plug in my power inverter to the cigarette lighter port, a built-in fan starts up to keep the internal electronics cool. Are you sure it's not that?