Loud rumbling noise inside gas

SOURCE: We aare in a home unit with a Rheem 80 gallon

1) The pressure release valve is operating correctly and the sound it made is typical.
So pressure release is okay.
Always protect yourself from very hot water that can spray out of pressure release.

2) Rumbling sound is typical sign of sediment build-up inside tank, and usually indicates that tank needs to be cleaned out.
Seems odd that a brand-new tank could have sediment after only 8-9 months.
Did somebody recycle a 'new-looking' heater and sell it as 'new' heater?
Look at label on side of tank, and find brand and serial number to find actual age of water heater:
http://waterheatertimer.org/pdf/WaterHeaterSerialNumberDecoder.pdf

3) Because sound might not be sediment.
You can add a comment and it will send e-mail, and I will continue to answer.
No reason to be terrified of water heater. They are very safe, and homeowner can fix electric water heater same day for a few dollars.

4) Please listen to exactly where sound is coming from.

5) If sound seems to come from the pipes, and pipes are vibrating, then the plumbing is suspect, for example a check-valve, or pressure valve, and we can talk about that when you add a comment.

6) If sound is from inside the tank, then I suggest you turn off power, let water cool, drain water heater, open bottom of water heater, remove element, and check for sediment.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html
http://waterheatertimer.org/How-to-clean-sediment-out-of-electric-water-heater.html

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