How to fix baseboard header periodic buzzing
Check the contacts in your thermostat and in the relay for your heater. It sounds like they're not opening completely when the desired temp is reached. If it's an old mechanical thermostat with the mercury switch, look for slag floating in the switch, in which case you'd want to replace it. If this is a rental unit, check with the landlord before doing any work and see if they'll correct the problem.