It sounds like your ducts could be harboring a buildup of debris or even mold, especially if the smell is persistent and comes back each time. I had something similar in our previous home. Learning
https://comfortclean.com/how-to-clean-a ... omeowners/ how to clean ducts in a house made a big difference. Once we removed the dust and particles inside the ductwork, not only did the smell go away, but the air quality noticeably improved. If you're not ready to hire a professional just yet, you can follow a step-by-step guide to check and clean them yourself. Just be sure to wear a mask and use a vacuum with a HEPA filter. Ducts can get surprisingly dirty over time, especially if you have pets or live in a dusty area. It's not a glamorous job, but it's worth it for peace of mind.