I installed a new block heater in my truck about two weeks ago. I noticed an audible buzzing from under the hood of my truck today while out clearing snow. It went away as soon as I unplugged the block heater. As far as I can tell it's still working and it's not popping the breaker on the house. Is it safe to plug it back in, or did I get a bad one? I've never had a block heater buzz before, anyone experienced this?