The treadmill is about 12 years old. It is a commercial treadmill and receives light use. I have had it from new. I have had this problem for a couple of years now. I have very recently had the electrics checked in the house and all seems to be fine.
The treadmill is properly grounded.
It is plugged into its plug socket with no other appliance using the same socket.
The problem takes place when the motor is on or when the treadmill is turned on. I do not leave it switched on.