First toilet flush of the day making strange noise

[Adsibob]

For the last month or so I’ve noticed that the first time I flush the toilet, there is a strange noise. It’s a bit like water is struggling to pass through a pipe somewhere, and so the water passes intermittently as it fights with an air bubble or vacuum somewhere. It lasts about 2-3 seconds and always sounds the same. It only occurs when the toilet hasn’t been flushed for 8 or so hours. After the first flush, the noise doesn’t reoccur in subsequent flushes unless you leave it for about 8 hours. And even then it won’t always make the noise, just usually.

All our toilets are wall hung with the cisterns inside the wall. Three are grohe and one is Vitra. Although I have only observed the sound with the Grohe toilets, I don’t think it’s to do with the toilets themselves, as if that was the case wouldn’t the issue occur with every flush, not just the first flush of the day? The Vitra toilet is the only one on the 2nd floor, whereas the three Grohe are on the ground and first floors. The toilets are also not that old, everything having been brought and installed in late 2021 and only in use since May 2022. 

 

I had considered it might be to do with the fact that I had switched off the circuit which runs to our two outdoor taps for the winter, as this issue started a couple of weeks after I did that this winter, but this issue hasn’t occurred in the two previous winters we’ve lived in the house and yesterday I switched that circuit back on and today we are still experiencing the noise. 
 

Another systemic issue might be our sewage stack outside. I shall post a picture in case anyone spots an issue, but again, I don’t think this is the culprit as it wouldn’t explain why the issue only materialised 32 months after we moved in.

 

 

[Adsibob]

[Adsibob]

I think technically the top of the stack on the left should rise higher than the roofline, but given it’s pretty high up, I’d be surprised if there is insufficient air flow up there. Is the  one on the right ok?

[Marvin]

Its difficult to tell the fall on the horizontal pipes form the picture. Horizontal pipes should fall 1cm down in 40cm along. I add additional fall for each 90 degrees of horizontal bend to overcome the redirection resistance.

 

The short stack on the right has a air admittance valve on it so it lets air in and not smells out.

 

Does the stack on the left service another loo?

 

 

 

[ToughButterCup]

  On 10/02/2025 at 07:56, Adsibob said:

 Doctor, doctor..... For the last month or so I’ve noticed that the first time I flush the toilet, there is a strange noise. It’s a bit like water is struggling to pass through a pipe somewhere ...

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Well, hmmmmm . Just pop up on the couch would you please - knees to your chest

 

@Adsibob, it gets us all in the end mate.....

[Marvin]

  On 10/02/2025 at 09:31, ToughButterCup said:

 

Well, hmmmmm . Just pop up on the couch would you please - knees to your chest

 

@Adsibob, it gets us all in the end mate.....

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Stop taking the pyss.  He's too young for that yet.... 😂

Edited February 10 by Marvin

[ProDave]

To get a serious answer you need to determine is it the water exiting the toilet that is making the strange noise?  Or is it the water refilling the cistern after flushing that is making the strange noise?

[Nickfromwales]

  On 10/02/2025 at 07:59, Adsibob said:

Is the  one on the right ok?

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Yes, but is completely redundant as the rising stack is vented to atmosphere. There was no need for a T and a short stack there, even less an AAV.

 

The whole stack looks as it should function perfectly, so the issue isn't out there  

 

Are the WC's gravity fed or cold mains fed?

[Adsibob]

  On 10/02/2025 at 11:42, Nickfromwales said:

The whole stack looks as it should function perfectly, so the issue isn't out there  

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That’s reassuring, so you wouldn’t even bother checking the fall of the flat-ish looking piece? Easy enough to do, they only issue being I’m not sure my ladder is tall enough to reach.
 

  On 10/02/2025 at 11:42, Nickfromwales said:

 

Are the WC's gravity fed or cold mains fed?

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Cold mains fed, I assume, on the basis that there is no cold water tank.

 

 The water comes in from outside where it is divided into two:

1) drinking/gardening water

2) everything else.

 The “everything else” then goes up to the second floor where it gets passed through a water softener, this then goes everywhere else.

[Adsibob]

  On 10/02/2025 at 11:01, ProDave said:

To get a serious answer you need to determine is it the water exiting the toilet that is making the strange noise?  Or is it the water refilling the cistern after flushing that is making the strange noise?

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Presumably, the only way to check this is to shut off the mains water supply, and then flush the toilet? That would eliminate the source of the sound being water refilling the cistern? The reason I’m not sure that’s the cause is that process shouldn’t be any different whether one is flushing for the first time in 8 hours or 8 minutes. But maybe I’m overlooking something.

Edited February 10 by Adsibob

[Nickfromwales]

  On 10/02/2025 at 12:47, Adsibob said:

so you wouldn’t even bother checking the fall of the flat-ish looking piece?

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Nope, but on reexamination it looks like the boss for the smaller waste pipe is leaking?

[Adsibob]

  On 10/02/2025 at 12:55, Nickfromwales said:

on reexamination it looks like the boss for the smaller waste pipe is leaking?

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Thanks @Nickfromwales, do you mean this:

 

If so, surely a small leak wouldn’t be causing this kind of flushing noise issue, particularly because the noise happens from the ground floor toilet as well, and that toilet connects to the stack below this point?

Edited February 10 by Adsibob

[Adsibob]

  On 10/02/2025 at 09:04, Marvin said:

Does the stack on the left service another loo?

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The stack on the left services a loo on the top (second) floor. The horizontal bit shown at first floor level receives waste from two toilets; I believe both first floor toilets send the waste to the right side of that horizontal.

[Nickfromwales]

  On 10/02/2025 at 14:07, Adsibob said:

Thanks @Nickfromwales, do you mean this:

 

If so, surely a small leak wouldn’t be causing this kind of flushing noise issue, particularly because the noise happens from the ground floor toilet as well, and that toilet connects to the stack below this point?

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Yes, just noted it, no reference to your noise issue. 👍

[Adsibob]

I managed to capture the sound on camera. Have a listen from about second 18 of this video. Any ideas what the reverberating sound is?

 

IMG_3542.mov

[marshian]

Well it’s new first for me - I’ve actually listened to a sound clip of someones toilet

 

What an amazing time to be alive

 

On a serious note sounds like an air emmitance valve being activated - you get a oomph type reverb as a slug of water causes a vacuum and pulls it open

[Nickfromwales]

  On 14/02/2025 at 07:12, Adsibob said:

I managed to capture the sound on camera. Have a listen from about second 18 of this video. Any ideas what the reverberating sound is?

 

IMG_3542.movFetching info...

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Can you do the same with the cold main shut off, and the cold tap on the kitchen sink open wide, please.

 

  On 14/02/2025 at 18:47, marshian said:

Well it’s new first for me - I’ve actually listened to a sound clip of someones toilet

 

What an amazing time to be alive

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This is the path we've chosen......best to embrace our final destination......