I have the same problem with several of our machines. The wiring is good on all, and new on one. Yet I get a slight buzz from them. This is more prevalent in the basement than upstairs. Some of the machines only do this in the basement, the others are more noticeable down there.
I do not understand this at all. Unless it's because I'm in an area literally surrounded by metal AC/Heater ducting. All of the outlets I use in the basement are heavy extension cords with self contained circuit breakers plugged into a grounded outlet.
I'd have the machine checked out too just to make sure.
Joe