I just want to say thank you for the advice given from a plumber from Taylor and Sons. I think his name is Damien.
I have this gravity fed hot water service above my ceiling in the apartments and was making noise at midnight for perhaps at least a year (since I moved in).
I have had a plumber from the body corp checking on those heaters more than once within months. All I was told is that they are all ok and was not able to suggest much about the noise. I was not convinced!
I did some internet research, reading a lot and found Taylor and Sons websites given some info about gravity fed hot water service.
On Monday morning I called up Taylor and Sons and the plumber helped me to diagnose over the phone the noise problem that I have.
The plumber suggested that it could be overheating and could be a heater belongs to my neighbour downstairs (because when we talk it through it appears my hot water didn't seem to have overheating problem). He suggested that we should get an electrician.
Everything he said was spot on!
I got an electrician came in within the next few days and he had replaced the thermostat of the heater that belongs to downstairs. Everything was fixed within a week for a year long problem!
Now no more noise! Absolutely fantastic!
Thank you so much for the advise.