ᐅ Noise Transmission Through Drainage Pipes and Rainwater Downpipes

Dear Forum,

I have the following issue with my small sewage lifting station and my home drainage system. If this is the wrong forum, please feel free to move my post accordingly.

The small sewage lifting station is located in the basement of my detached house (built in 2019). When the lifting station pumps out, I can hear gurgling noises in the bedroom on the top floor. The sound transmission does not occur inside the house but through the main drain pipes and the rainwater downpipe outside. The noises are loud and disturbing. Therefore, I would like to know what I can do about it.

Here is a sketch of the home drainage system:

A drain cleaning company has already conducted a camera inspection of the affected pipes and lines. However, no irregularities (e.g., excessive slope) could be detected. My general contractor said he has never encountered such a case before. Although he confirms that noises of this volume are unusual, he does not know how to solve the problem.

My questions:
- An acquaintance told me that rainwater and wastewater are only allowed to be combined at the inspection chamber. Is this correct? Could this be the cause? As shown in the attached sketch, rainwater and wastewater are already combined in the main drain pipe in my case.
- What other causes could there be for the noise? What measures could be taken to reduce it?

Thanks in advance for any helpful advice.

Best regards
timmy-baut