My air quality is terrible, could the culprit be panned joist returns?

I have paid to have my ductwork cleaned and sealed. It helped for all of 2 weeks, now it's filthy all over again. I've been told my system itself is top notch, I've cleaned debris off the condenser outside and I ALWAYS change my filter early. There is a strange smell that comes from the supply vents, and I smell it strong when the AC or heat first kicks on. I'm NOT going to attempt any of the repairs on my own! I just want to know what the problem is before I hire someone to fix it. I'm only 23 and I don't have a lot of money :/ i noticed I have panned joist returns in the walls which are full of debris every time I vacuum them, and there's always crumbled pieces of sheetrock in there. Sometimes, I hear rattling in the biggest return in my living room which I suspect is debris. I also smell that weird smell in the returns when grilles are removed. My house is excessively dusty although I have 2 HEPA filters running 24/7 and I vacuum with a HEPA vacuum once or twice a week... Could sealing the panned joist returns fix my issue? If so, what is the best way to accomplish that?


It's worth noting that I'm suffering considerably. I have severe asthma and it's progressed to the point of diminishing my quality of life. Please help

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