I am installing a Marley series 2500 baseboard heater (220V) with Marley single pole thermostat mounted at the heater. The heater works but there is an annoying buzz when it is heating. Is this inherent in the heater, or have I connected it incorrectly? Double size breaker, through switch at wall to thermostat in unit.

Take a look at this previous post:

https://www.askmehelpdesk.com/electrical-lighting/baseboard-electric-heater-buzz-hum-45561.html?highlight=baseboard