We have a two story house and the upstairs toilet causes a loud thud in the walls after flushing (not always but at least a couple times per day). It's definitely not the water line and is related to the drainage. I've opened the walls all along the pipe and the drain pipe isn't loose or hitting any part of the wall or studs. I can't even tell exactly where along the pipe the noise is coming from. I'm stumped as to what it could be. Any help is greatly appreciated.

New sound or old sound? How old is the house?

Let me know... Mark

New sound. It's about 2 months old. Finally got around to looking into what is it. House is 15 years old.

Hey hey...

My guess at this time is that you may be experiencing a WATER HAMMER...

Here, you may have water hammer "bumpers" (see image below) behind the wall that have filled with water over the years and can no longer absorb the reverb associated with the toilet water supply.

To test this, you would need to shut the water off to the house and open all valves in the house to drain the system. Now, you can wait a half hour to allow air to displace the water in the bumpers or you could disassemble the toilet shutoff which will speed the process.

Once everything has drained you will want to refill the system and see if the bumpers have been "recharged" with air and are no longer making noise. If this works, all "bumpers" should be recharged throughout the house... ;)

If this fails to work I'll need more information like when the sound happens... at beginning of flush or at the end, for example, OK?

Try the quick drain/refill. Remove aerators at the sink faucets if you notice a decrease in water flow when all is done.

Back to you...

Mark

Thanks for the ideas but I previously had water hammer and fixed it - both by draining the lines and by securing a loose pipe. This is completely different. It happens after flushing while the toilet is still refilling (and water running).

If it's happening while the toilet is filling then I might next suspect a loose washer at the fill valve or it could be a loose washer at the angle shut off below the toilet... sound reasonable?

Mark

An additional note is it only seems to happen when there's something in the toilet - toilet paper or otherwise. It doesn't seem to make any noise when just flushing water by itself.

This is a new toilet and the old toilet was making the same noise before we changed them. Is this washer part of the toilet? Or the line attached? As mentioned below this only seems to happen when there's something in the toilet - not just water

This is a new toilet and the old toilet was making the same noise before we changed them. Is this washer part of the toilet? Or the line attached? As mentioned below this only seems to happen when there's something in the toilet - not just water

Waste matter in the toilet bowl shouldn't make a difference in any way... thinking it may just be coincidence.

Without hearing the noise I still stand by checking the washers in the fill valve and the shut off to the toilet and see if that improves things, OK?

You might also lift the toilet flapper and be sure nothing is stuck under the flapper as I have seen this make a few different noises over the years... ;)

Mark

I don't have a flapper I don't think. This is a dual flush toilet. I also am not sure where to find the washers for the fill valve. Any idea?

I just checked and it seems the bang is happening at the moment the contents of the toilet hit the 90 degree bend in the basement (it drops straight down from the upper floor). Nothing is loose, so I don't know why there would be a new thud noise.

Also tried flushing with the water turned off at the toilet and it's still making the same noise.

Unfortunately, you got me, too! I have yet to install one of the newer dual flush toilets so I cannot advise on these... still thinking that the fill valve is the issue... almost always is!

I would give a quick call over to the toilet manufacturer and speak to the technical department... usually pretty helpful guys/gals at these places.

My only other thought is that if you really think this is related to the drain line you can lift the toilet and take a peek inside the pipe and inside the underside of the toilet bowl... *shrugs*.

Please keep me posted on anything you find out... curious one, for sure!

Finally, give a few of the other guys on the plumbing page a chance to chime in. Milo (another "expert") or some of the other guys may have worked with these dual flush units.

Thanks...

Mark

Thanks for your help. I think I'll wait to hear if anyone else has an idea before I pull the toilet up.