What type of water supply do you have ?
city or a well ?
I am just asking this to see if possiibley there could be air that you are hearing
Is your water heater gas or elec. ?
There are 2 elements in a water heater and that's about it
If the upper one was possibly not completely covered in water i could understand a sound coming from it
This is very doubtful because the water coming out comes out of the top and air can't be trapped in there
The only other thing i can think of is that the pump on your well could be filling it up with the pre set pressure
It could be just a coincedence that you happened to be standing there when it came on this would explain the 2 minutes or so you spoke of