Pungent smell from heating circuit distributor

I guess from the type of smell it's the same as Joedreck. That's how decomposing animals smell. If it's not found in the pipe - are there shafts for windows, inspection hatches nearby, is there any “Gerümpel” lying around that hasn't been looked under yet? A dead mouse and it smells terribly.

So there is nothing in the utility room. However, we do have a cat. There is no basement, and the area around the utility room on the property is only paved. There is no junk lying around. But thanks for your ideas. The landscape gardener has announced for 2:00 PM, then the inspection shaft will be opened and cleaned with a pipe cleaner. If that doesn't help, I will call a professional pipe cleaning company on Monday that has the option to inspect the pipes with a camera.