Ground floor flat - intermittent sewer smell

[Radders_72]

Hi All,

I live in a ground floor flat and sometimes when the back door is open get a sewer smell come into the hallway. It's very intermittent and only happens when the upstairs flat is running water. There's two drains from upstairs, one from the kitchen and then one from the bathroom which is located next to the back door. Any ideas what might be causing it?

Thanks in advance.

 

[Aquarius]

Could be a crack somewhere?

 

[king of pipes]

Any man hole covers outside ??