ᐅ Bathroom smells like a drain

Hello, about two years ago our house was handed over, and shortly after, an unpleasant smell started to appear in the bathroom from time to time.

At first, I didn’t pay much attention to it, since during the drying phase it was normal for rooms to occasionally be damp. But now, honestly, this smell has been bothering me for months.

It can be detected near the washbasin and more recently also in the shower and bathtub right by the drain. A slightly rotten, musty, or moldy odor fills the entire bathroom after a while and spreads throughout the whole house (bungalow) when the door is open.

The plumbers have already been, but they couldn’t find anything. The trap at the sink is working, also in the shower – nothing is getting siphoned out. The drain piping at the washbasin has even been replaced.

Over the weekend, I also thoroughly cleaned the flat shower tray using cleaning agents… but the musty smell was back yesterday.

I really don’t know what else to do. I have already gone through various forums and so on.

An extensive cleaning, with or without chemicals, has unfortunately not helped so far.

Do you have any ideas?

Could it possibly be related to the roof ventilation?

Can sewer gases rise back through the trap water due to overpressure or underpressure?

Regards