just replaced my heater in my tank with a new one, after a while i noticed a knocking noise coming from the tank, and at first couldnt figure out what it was. after a while i decided to unplug the heater to see if it might have been that, and the noise stopped. is this normal for a new heater, as i dont remember this happening with any of my other heaters. the new heater is a fluval M300, thanks in advance for any help.