There will always be a little bit of metal expansion noise when a baseboard heater goes quickly from totally cold to maximum hot ...... but it can be minimised if the grommets or plastic shields are in place between the heating element and the metal that it sits on. Making sure that these are in place and that the heating element moves freely will seriously reduce the noise.
Having said that .... perhaps the problem may actually be caused by the thermostat.
Recent electronic thermostat developments have actually totally eliminated this noise.
Old style thermostats go on ..... stay on a while then go off ...... then stay off a while. This allows the element to swing from room temperature to very hot and back constantly. It also makes baseboard heaters the least comfortable of all heating systems.
The electronic thermostats .... set-back or otherwise .... have changed that. The better of these devices actually check the temperature of the air every 3 seconds or so ...... and by proportional control modulates the outpiut.
They are turning the baseboard on or off .... or leaving it on ...... or leaving it off .... every 3 seconds. The effect of this is ..... it doesn't get hot .... it just gets warm. If you do this constantly you could maintain any temperature you want and it never gets either cold or hot.
So under normal circumstances the electronic thermostat causes the baseboard electric heater to stay warm ..... just the temperature necessary to keep the room at the set temperature.
If it gets colder outside it will stay on a little more to maintain that temperature .... it will be slightly warmer but still constant.
This process not only saves a lot of energy and keeps the room more comfortable .... it eliminates that expansion and contraction ping in electric heaters.
The same heaters ..... with a better control and quiter as well.