Ignition buzz

My 2013 Tacoma's ignition buzzes when I turn the key to turn the truck on. It doesn't happen all the time but has recently happened a lot more than usual. The buzz does not happen when the truck turns on. There have been a few instances where I turn the key and it buzzes so I let off (stops buzz), then turn again and it buzzes again. I let off and try again and turns on with no buzz. The past two scheduled maintenance visits I have had the Toyota service department look into the problem and, of course, it doesn't buzz for them and they don't find anything wrong. Has anyone experienced this? Any guesses?