If you took off the cover you would probably find dirt, gravel, grinding shavings or some loose metal ect: hanging on the transformer thats viberating from the magnetic field.
Some machines use a secondary sliding core inside the primary coil to control amperage that gets loose and viberates. There's plastic guides to adjust to help with this problem.
Inside the cover is high voltage so be very careful even after you unplug the power to the machine.
On some machines the cooling fan is thermostaticly controled and it takes some time for the fan to come on.
The smaller 115v machines usually have a continous running cooling fan.
I would order a service manual from Miller as then you will know what you have.