It is the annual change of your filters.
I change my filters once every 4 months because I like to stay up on that maintenance.
I have a pile of brand new modern ones which I opened in a stack on my basement floor. It’s easily about 3 years worth. I open them because when I do the change, I do not want to spend time opening them and in this way I can quickly do the change. I used to have an HVAC expert come to my beach house and do this but he gave me a lesson on how to do this myself so I can save money. I always loved that about the HVAC company around here. They always went above and beyond to look out for their customers. The last few afternoons everything in my beach house started to smell so bad. I didn’t know where it was coming from but then thought maybe something died and maybe it was in my heating vents. I had to call the HVAC company and they sent out a heating and cooling tech. After he examined it, he said the smell was dog pee. I was confused at first but my dog Ash wouldn’t have done that, would he? I checked the pile of air filters on the basement floor and they all smelled. It was disgusting and I was disappointed with Max. The HVAC worker substituted the air filters I had and said I should store them somewhere else going forward. I learned an important lesson that afternoon. Do not leave your air filters on the ground, you never suppose if your dog will pee on them.