Talk About freaking a guy out!
I had this same thing happen to me around a month ago.
I bought a 55 gallon aquarium, Built the stand for it, Set it up, and got it cycling.
After letting the heater acclimate I plugged it in and within an hour I started hearing clicking sounds.
At first I thought it was the stand I had just built.
Then I thought it was the new tank creaking.
I kept an eye on the level of the stand making sure it was not failing, and kept checking for cracks in the tank, but nothing seemed to be physically wrong.
I bought a bigger heater a month later cause I thought the one that came with the tank was running a lot and was not doing the job as well as a bigger one could.
So I put the old one into my smaller tank to replace the cheaper "auto" thermostat heater.
Then little tank started clicking then it all became clear!
That @#*!$& heater has been keeping me up at night with every darn click, making me think that my aquarium was going to turn my mattress into a water bed