Thank you for your reply, I'm just writing an update in case anyone
had a similar problem.
I hired a professional and he found multiple sources for the odor
problem. The drainage pipe was clogged but it wasn't the main
culprit. The smell was coming from the crawl space because the return
duct that passes through it was damaged and it was taking air from the
crawl space rather than from the inside of the house only. I was told
I should consider installing an insulator on the bottom of the floor
separating the crawl space from the living area because it was
contributing to the odor. Sadly, for me, that isn't an option
considering it's very costly and doesn't guarantee to solve the odor
problem. For now, the drainage pipe is fixed and I will seal the
return ducts soon and hope that would be adequate.