Hard water sediment build up - RELIANCE ELECTRIC WATER HEATER

Sounds like build up of sediment
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

Copy following link for element resources:
http://waterheatertimer.org/How-to-replace-water-heater-element.html
You can soak element in white vinegar to remove scale, and that will reduce some of the whistling noise.
Cleaning sediment out of tank can restore full efficinecy to electric water heater:
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html#electric
Another cause of water heater noise are factory-installed heat traps.
http://waterheatertimer.org/New-water-heater-making-noise.html
Add a comment for followup

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

Rumbling sound usually means sediment build-up inside tank.
Yeas a leaking water heater is dangerous because it can cause water damage.
Copy following link to determine source of leak.
http://waterheatertimer.org/Leaking-water-heater.html
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

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

Marathon elements are specific to that water heater and can be ordered online, but generally are not at the local hardware.
Good to keep a couple spares around.
Marathon is 240Volt water heater.
Thermostats turn off one leg of the 240Volt circuit, so elements and themostats will always test for 120volts.
You removed wires off elements and tested each for continuity and tests showed elements to be good.
Test top two screws of upper thermostat and make sure you are getting 240V to the water heater.

If water heater has some hot water, then that says water heater is getting 240V, upper element is good, and remaining suspects are bad thermostats, bad lower element, or sediment build-up, or combination.

Open following link for troubleshoot run-down:
http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html

Water well.
If you have sediment build-up that reaches lower element, then water does not circulate around the element and water heater no longer heats very fast or accurately. If sediment is the problem, sooner or later the lower element will burn out.
Doesn't hurt to flush out the tank now and then, and vacuum out the sediment if that is problem.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

You posted under electric water heater.

1) Gas or electric, the noise is from sediment build-up inside tank.
Once sediment builds up, the only way to solve problem is to clean the tank out.

2) To restore full efficiency to electric water heater and stop the noise, clean the tank:
http://waterheatertimer.org/How-to-clean-sediment-out-of-electric-water-heater.html
3) To clean gas water heater is more difficult and less successful:
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

4) To prevent sediment problem, flush water heater each 6 months before sediment can build up.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html#flushelectric

5) Other things to know for water heater maintenance.
http://waterheatertimer.org/Replace-anode-rod.html
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html

The crackling noise is from build-up of sediment.
I forget exact reaction that causes the noise.
Water heaters exposed to hard water will create sediment inside the tank.

Following link shows sediment and remedies for gas and electric water heaters.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

There are two water lines connected to water heater.
One is marked Hot and the other Cold.
The shut off valve is on the Cold line.

To eliminate sand and dirt from entering your water heater, the 'sediment' filter should be installed on Cold line.
Sand and dirt are sometimes called sediment, but this type of sediment is not the same as calcium carbonate sediment that accumulates inside a tank that receives hard water.

When you have hard water, then water heater makes calcium carbonate 'sediment' deposits that accumulate in bottom of tank. A 'sediment' filter will not prevent this problem.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html
http://waterheatertimer.org/How-to-clean-sediment-out-of-electric-water-heater.html

If you have hard water, to prevent calcium carbonate from forming inside tank, you need a water softener.
Water softener will prevent calcium carbonate sediment forming inside tank.
However water softener will shorten life of water heater anode rod.
http://waterheatertimer.org/Replace-anode-rod.html

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on water heater.

'Rheem 81VP15S point of use' is a single-element, 15 gallon, 120V electric.
Other 'point-of-use' sizes range from 2-1/2 to 6 gallon to 20 gallon.

Electric water heaters have 2 parts: thermostat and element. One is not functioning correctly. Most likely the element caused by sediment build-up.

How to determine if tank has element or thermostat problem:
Do not turn off water going into tank. Put hose on drain valve. Open drain valve. Does water come out like open spigot? If water does not flow, then heater is likely clogged with sediment.

Solution 1: If water flows out like a spigot, the thermostat is probably bad. Read: How to replace thermostat.

Solution 2: Water does not flow, so element is surrounded by sediment build-up and cannot heat water. You need to clean out tank with shop vac, and put in new element ... plan full day and 3 trips to hardware store. Simply draining tank will not remove sediment. Sediment is chunky and will not come out voluntarily.

Buy element before starting:
-Turn off electricity
-Open cover on side of tank, remove insulation (these have to be put back for proper operation of thermostat)
-The are 2 electrical parts >>the element and the thermostat which is located above element
-Element wattage is printed on element and also shows on label on side of tank
-I think Rheem element is held in place with 4 hex bolts
-Take photo of element to hardware store so you have part on hand before removing old element
-If element is 1-1/2' diameter hex type, you need inexpensive element wrench from hardware store + the element

Drain tank and clean sediment:
-Read: To open drain on water heater so water comes out
-Read: How to clean sediment out of water heater

I assume your is an electric hot water heater. Is your water "hard" in your area? If so, it would be worth a try. What usually happens in a heater your age is that sediment builds up causing the lower element to overheat and burnout, causing less hot water to be produced

With Rheem you have to back to the installer.Most of the heater have ten year warrany.