Noisy Rheem 16 instant hot water heater

Probably the factory-installed heat trap nipples are clogged.
Open following links.
http://waterheatertimer.org/Low-hot-water-pressure.html
http://waterheatertimer.org/Low-hot-water-pressure.html#trap
Noisy whistling and humming and low pressure are common problem with heat trap nipples and inserts from factory.
Replace heat-trap nipples with pex-lined nipples, or remove heat trap inserts with screwdriver or needle nose... careful not to break insert so it falls into tank ... which can then get lodged in hot water line elbow or under-sink shut off.
Factory-installed heat traps are part of overall strategy so manufactures can claim more efficient water heaters, without requiring customers to any thinking.
Other strategies include foam insulation instead of fiberglass (so that wires cannot be replaced on electric units), reducing factory-set temperature from previous 130-135 to 100-120, and re-estimating downward the hot water use for average family.
So some of the strategies are useful, while others problematic or imaginary.
Call manufacturer to pay-for or provide new pex-line nipples. Warranty for fist year covers Labor and parts, and after 1st year, warranty covers parts
http://waterheatertimer.org/Water-heater-manufacturers.html
Save $10 a month in future replacement costs: maintain water heater so it lasts longer:
http://waterheatertimer.org/How-to-maintain-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Before calling service technician, open following links for Rheem troubleshooting resources including error codes.
Error codes are considered start-point for troubleshooting.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
http://waterheatertimer.org/pdf/ErrorcodeTroubleshooting.pdf

Error 11 is no ignition.
This problem can probably be solved by routine yearly maintenance shown in instruction manual which recommends: Yearly cleaning, adjustment and inspection of burner and combustion parts.
The yearly maintenance would check incoming gas supply, venting, wiring and grounding, and burner and ignition parts.
All gas water heaters today require same maintenance: tankless require yearly, and tank-type require periodic (1-3 years)

Read following technical bulletin from Rheem:
http://waterheatertimer.org/pdf/Noisy-gas-water-heater.pdf

Find age of water heater:
http://waterheatertimer.org/pdf/WaterHeaterSerialNumberDecoder.pdf

There is no valve inside the tank.
http://waterheatertimer.org/How-it-works.html
The constant noise sounds like water heater is running full time, except same sound after electric was turned off. Is there a check-valve nearby? What happens when you open-close shut off valve located on cold water line above tank?
Is there a water leak inside house, or is TP valve running water into a drain line?
http://waterheatertimer.org/How-to-clean-sediment-out-of-electric-water-heater.html

Post question on professional water heater forum
http://www.thetankatwaterheaterrescue.com/forums/forum3/

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For a price, expert works with you while you work on water heater or any do-it-yourself project.
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it may be the thermal coupling that is bad for the pilot light. if it goes bad the flame will go out. replace thermal coupling. so if the pilot light is out then it will not heat water. or turn burner on.

This happen when there is air in the gas line...if the problem is still there after a couple of days you will have to return the unit for an exchange the gas control is giving problem...contact rheem first before making a exchange decision..they will walk you some step first to ensure there is not a problem

valve is bad boy a new one and install

Check the gas valve first, it may have inadvertantly been turned down. The knob should be red and have settings on it. If it hasn't been turned down, it sounds like it's nearing its end. Good luck

There could be blockage in your water heaters venting on the outside of the home. It sound like the high limit switch has been tripped. Try unplugging the tank and plugging it back in. this will reset the tank. if the problem persists you will want to call your provider for service.

The dip tube which carries the incoming cold water to the bottom of the tank is bad/gone. If you can find a rheem dealer you MAY be able to get a new one but it's probably time for a new water heater

On the back of the gas valve (where you turn the temp up) there is a stem that actually goes into the tank that senses the temprature of the water it could be dirty or buried in sediment depending on how old the tank is, or just bad. if the tank is over 10 years old i wouldn't bother replacing the gas valve, the tank is probably not far behind. A new valve will run around $100.00 maybe a little less. You wont find it at menards or home depot in stock but they may be able to order it for you. Menards sells rheem products or try you local plumbing supply house. REMEMBER TO RELIEVE THE PRESSURE ON THE TANK BEFORE YOU WORK ON IT. IF YOU CHANGE THE VALVE DRAIN IT FIRST!