So I plugged in the block heater tonight for the first time ever and within 20 seconds or so of plugging it in the front of the truck starts making a vibration noise and you can feel it if you put your hand on anything in the front of the truck pretty much and hear it coming from the radiator. Is this normal or should I unplug it. It's a 1989 F250 ext. cab 7.3/zf5, 4x4 all stock. Thanks in advance Jeff