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Old 09-22-2021, 02:36 PM   #1
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Buzzing sound when flushing black holding tank

We have a 2506s and as usual after dumping our black holding tank, I flushed or rinsed that tank. Connected my rinsing hose to the port on the outside of the trailer and turned on the camp ground faucet. After a few seconds a relatively loud buzzing sound started. It was coming from the corner of the trailer at the port. I could also feel vibrations when placing my hand on the outside wall near the flush port.

I assume there is a check valve there and it has started to malfunction. The buzzing would stop for a short time now and then. When that happened I couldn't hear the water running into the tank so that definitely needs to be fixed.

I'm guessing the check valve is plastic and not as robust as one for homes. Should I replace the check valve with an identical part or is there a more robust replacement that would be plug & play?

Thanks,
Mitch
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Old 09-22-2021, 02:44 PM   #2
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It could be the sprayer in the tank. I think they spin. My has more of a noise than a vibration.
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Old 09-22-2021, 04:03 PM   #3
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It could be the sprayer in the tank. I think they spin. My has more of a noise than a vibration.
Thanks for the reply.

The buzzing is definitely coming from something that is next to where the hose connects at the back and not the tank under the trailer.

Next time I will listen to see if there is a spinner in the tank.
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Old 09-22-2021, 04:15 PM   #4
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If it's a factory installed flush system it could be the water passing through the vacuum breaker/check valve. If so, not an issue. Mine will sometimes make that sound depending on water pressure applied.
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Old 09-22-2021, 05:40 PM   #5
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If it's a factory installed flush system it could be the water passing through the vacuum breaker/check valve. If so, not an issue. Mine will sometimes make that sound depending on water pressure applied.
Same here. If water is going into the tank from the flush hookup, and running out at the dump hose, without any water ending up inside the RV, all is good.

Ignore the noise. It will come, go, and change how it sounds, with water pressure.

Could be chatter on the backflow preventer or noise from the spinning spray nozzle. All normal.
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Old 09-22-2021, 05:55 PM   #6
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Spring loaded diaphragm in the back flow valve at the water connection.
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Old 09-22-2021, 09:36 PM   #7
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Spring loaded diaphragm in the back flow valve at the water connection.
The diaphragm in the vacuum breaker-check valve is free floating. There is a spring loaded ball check valve on the output side of the valve. The vacuum breaker (opening to ambient) is supposed to allow the diaphragm to fall back down to seal off the valve input side as the residual water in the valve cavity drains back out through the input port (hose connection).
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Old 09-23-2021, 01:57 PM   #8
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Black tank flush buzz

We also have a 2506S and I also noted the buzz/vibration when using the black tank flush.
In my case, I checked the connection inside the trailer by opening the corner cabinet door in the bedroom (the stupid one with three hinges - unscrew the single hinge side) where our black tank flush port connects on the outside.
The strange noise was caused by the hose - it was severely twisted, constricting the flow, and the flow through the narrow passage generated the noise, depending on the pressure (sort of like the reed in a woodwind instrument).
Loosened the connection on the port, straightened the hose, and the noise went away.
Worked for me - your situation may vary - good luck.

edit: Added benefit - significantly increased flow to the flush!
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Old 09-23-2021, 02:05 PM   #9
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The diaphragm in the vacuum breaker-check valve is free floating. There is a spring loaded ball check valve on the output side of the valve. The vacuum breaker (opening to ambient) is supposed to allow the diaphragm to fall back down to seal off the valve input side as the residual water in the valve cavity drains back out through the input port (hose connection).

GREAT EXPLANATION!
Thanks it helps me too.
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Old 09-23-2021, 04:51 PM   #10
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Back home now, did a rince and water flows as it should without any noise.

I also opened the cabinet in the back corner. Everything there seems to be ok, picture attached.

Thanks everyone for your replies with all the great information. Saved me a lot of trouble and expense. Much appreciated! Click image for larger version

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Old 09-24-2021, 09:59 AM   #11
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Originally Posted by MitchL View Post
Back home now, did a rince and water flows as it should without any noise.

I also opened the cabinet in the back corner. Everything there seems to be ok, picture attached.

Thanks everyone for your replies with all the great information. Saved me a lot of trouble and expense. Much appreciated! Attachment 263295

I am sure someone here mentioned but I don't want to read the entire post so I'll just through out this thought. Water pressure will effect how and when it buzzes. Seems to buzz with lower pressure. Buzzing is normal.
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