I have used electric multi-meter for many year (perhaps since I was a junior high school student)
This is the first time I buy and use a clamp meter.
After measuring, I confirmed that the heater was not always running with the maximum output (every night, we could hear "click" sounds from the heater ever 10 -20 seconds, and my wife thought that really noisy in the night).
From the clamp meter, I also the current changed from 8.5 - 9.0 A to 0.2 A about every 10 sec. This is the noise come from.
I just guess that the switching sound is using a thermo-switch to prevent the device overheating.
In addition, I think the life skills are more important than any grades of subjects in the school. Thus I taught my child how to measure AC current, even he did not know what was he doing!
Fluke AC/DC clamp meter manual (in PDF)
Electrical wiring in North America - Wikipedia