Lound vibration when running

When the noise occurs when there is no hot water being used, it's likely there is a vacuum that's siphoning water away from the unit and causing loud noise and vibration. ... Another common problem that can create noise in tankless water heaters is a dirty flow sensor which controls how much gas is sent to the unit.
https://www.youtube.com/watch?v=Fsb8Wx9PRvY

Never trust anything that comes to you new because usually they are made in china and are not consistent, but next time make sure clean air is being supplied if impeller gets dirt build it becomes imbalanced and will draw high amps and create that vibration noise. Also vibration noise can be caused by improper combustion set up.

My Navien is doing the same thing, and it's 6 days old. The older unit did the same thing as well... I'm guessing it has nothing to do with the unit, and something to do with the line... I'd love to get to the bottom of it as well.. :(

1) What brand and type water heater? Gas or electric? Tank or tankless? Or boiler? Solar assisted? Indirect? Wood-burning? Oil-fired? etc?

2) How old is heater? Is this a new installation? Or installation that worked before and vibration is something new.

3) Or are you talking about a washing machine instead of water heater? Or hair dryer? Or GMC truck?

4) Add a comment and give more exact information from label located on side of unit, for possible assistance.

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

I would check the metal door on the bottom near the burner, sometimes they come loose and vibrate. If not that then I would suspect the chimney liner inside the tank, I have seen them corrode and start vibrating when the gas is running

A low to loud vibrating noise is an indication of an Air /Fuel imbalance . When you flow more water , does the noise become louder ? if so , have a service technician check for pproper incoming gas supply . The requirements are located on the left hand side of the Rinnai unit . There will be a minimum/maximum specification.
Another issue could be the burners being "dirty" and needing to be cleaned .
Also investigate the venting and make sure all joints are sealed properly and the venting is sloped correctly . If you have your installation manual , please refer to the venting section in regards to usage of a condensate collector . This can be a cause of noise .
There is a clear vinyl hose running from the bottom of the heat exchanger to the front of the gas valve . Please inspect this hose for any moisture or signs of staining . This can lead to issues with the gas valve . It must be determined where the moisture is coming from (See previous comment about Condensate collector !) .
The cold water solenoid valve usually has nothing to do with this type of noise .
As far as yourself being able to repair the unit ? I have no idea of your experience but a qualified licensed service technician should be used as this is a gas fired appliance and as such is subject to huge liabilities when being repaired .
Have them call Rinnai Tech Support while on site for any assistance needed.
You shouldn't have to "put upo " with any noise issues . The Rinnai ujnits run at 49 decibals (about the same level as a conversation .
Good Luck !

I would check and see if the ventpipe that is coming off of the top of the motor is strapped to any duct work in the basement or touching and floor joyces. Even if the venting is touching the duct work a bit, it will amplify the vibration through out the home.

The only other issue it could be would be that the fan blades are bent and rubbing up against the inside of the motor causing the noise and extraem vibration

There are several different things the sound could be. If it is a popping noise it could be from the sediment inside the water heater as it is heating. It could also be noise(bang) from the water pipes having excessive pressure. If the water heater is gas it could be the noise from the burner when in operation or gas regulator vibration. Need a better description of the noise to be more helpful.

Sounds like there is something loose in the line.Like cut off valve,check valve.

If the water heater is working fine than I would not worry about the function of the water heater. What the problem may be is that some of your WATER LINES HAVE MOVED, AS THEY WERE NOT SECURED PROPERLY. The sound you may have heard was the Copper or PVC(Plastic) lines just getting back comfortable. I Suggest checking that your water lines are secured about every 3 or 4 feet with the proper ancjhor....Metal for metal pipe, copper for copper pipe, etc. Sometime as in my house, pipes are run through a hole drilled through a stud or a piece of wood, usually when hot water is run through this pipe. You can here an expansion of the water line interacting with the pipe expanding, if the hole in the wood was not drilled big enough......Experience of remodeling a very old house.......Joe