Could the neighbours be causing this smell?

[Worryallthetime]

I live in a terraced house and recently have noticed a horrible drain sort of smell at random times during the day. Smell lasts about 10 minutes each time I notice it. My neighbours toilet flush has something wrong with it, extremely loud fog horn sound everytime someone flushes. This is driving me mad as it, but it actually seems like the smell comes after they have flushed their toilet. Is it even possible for a smell from their bathroom to come into my home? Maybe it's a coincidence? The smell literally fills the whole of my house!

It sounds like the washer in their fill valve is in need of replacing. It hass probably got brittle and isn't allowing water to fully flow through it, which causes many different noises from a whining noise to a fog horn sound.
In regards to a smell though, it is likely to be a blockage in the drain somewhere. The smell is likely coming back up through your system

Speak to them and one of you arrange to have their toilet and the sewage pipe and drain looked at.

More likely to be a blockage in a drain in my property?

it could be a blockage in the drain/sewage pipe between both properties

How old is the house?

To get such a smell after flushing the WC suggests to ne a broken or leaking soilpipe or drain. This is quite common in houses built prior to about 1960, and IME extremely common in houses built prior to 1945.

There could be a blockage, but it shouldn;t cause your house to smell unless (e.g.) you've had an extension or conservatory built over the old drains and they are now inside your house.

Or it could be due to bad design and construction of a replacement or additional bathroom.

They haven't got a Saniflow, have they?

House was built in the 1930s. There was a kitchen extension put in my house many years ago, which was first used as a bathroom. The lady who used to live here was unable to get upstairs, so needed a downstairs bathroom. Could it be something blocked under the kitchen extension floor? We have a manhole out in the garden if that makes any difference. Saniflow? I have no idea...?

it's always interesting to look in a manhole. You can probably pry the lid up with a spade, and slide it aside (it might possibly have lifting eyes you can use)

It should be empty and wet inside, except when detritus is flowing through. It should not smell much. If you flush a WC, you should see the water flush through briefly, then stop. If you are in a terrace, you may see the drain from next door enter on one side, and to the other house going to the other house, and one or more channels coming from your house.

If your house was built in the 1930's, I would expect to see an iron soil pipe from the WC, and probably a smaller pipe from the bath and basin, running down the original outside wall of the house and going into what was once the ground behind the house. Inspect round these in your extension. Is that where the smell comes from?

forgot to say, if the fault lies in underground drains that serve more than one house (as is usual in a terrace) you can call your water/waste company to look, clear and maybe mend it. If the fault was caused by work in your house, or your neighbour's, it will be down to the homeowner. But if it just needs rodding and jetting, IMO the workmen will do it if you ply them with tea and doughnuts.

Thank you all for your help!

Thank you PigletJohn. No smell coming from there. Will try to contact the water/waste company.