Water heater knock, not while heating

All, I uploaded a youtube video where you can hear the sounds I am talking about (starts at the 9 second mark) http://youtu.be/xclW_U-r3ac . The water heater was not heating at the time (gas), and the popping only started when I turned on the laundry sink.

I am under the impression that sediment knock, which is what it sounds like, only occurs while the water is being heated; not when there is a demand for hot water.

Any help appreciated.


I think the water is under enough presser to not turn to steam .
You open the tap the pressure is lowered and now the high water temp can now turn to steam … Roy .
I expect if you leave the gas on it will eventually knock

Thanks Roy. The water pressure was slightly over 100PSI, so your explanation makes sense.