One viewer has tried loosening the screws that hold the heater to the wall to stop that ping noise, to no avail. No, that is not where it is rubbing. 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, as you see in the first picture. Making sure that these are in place, and that the heating element moves freely will seriously reduce the noise.
Recent technological developments have actually totally eliminated this noise: the electronic thermostat. Old thermostats went on, stayed on a while then went off then stayed off a while. That allowed the element to swing from room temperature to very hot and back constantly. It also made baseboard heaters the least comfortable of all heating systems. But 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. So they are turning the baseboard on or off, or leaving it on, or leaving it off, every 3 seconds. The effect of this is like turning on an electric range burner to hi for a couple of seconds, then turning it off. It doesn't get hot, it just gets warm. If you do that 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. They are quieter as well. The same heater, just a better control.