That doesn't sound right. There aren't any moving parts to make noise. It may be possible its from thermal expansion in the unit. But...
"AquaClear Compact Submersible Aquarium Heater offers accurate and reliable aquarium temperature. Its ceramic heater core allows for even and efficient heat distribution and its exclusive sealing system prevents humidity from entering the heater. The slim-line glass heater tube offers optimum heat transmission. Includes heater mounting brackets. For 20-50 gallon fresh or saltwater aquariums. 11.5" long. Shipping weight: 1 lb."
If that seal is broken moisture may be hitting the heater core and making that noise. Think old house hot water radiators. The old noisy monsters. I would consider removing it from the tank and getting a new one. At least contact the manufacturer to ask them about it if you are going to keep it.