We recently purchased a four plex, there is a mystery musty smell in the top two units and I have no idea where it is coming from.

We smelled around all the sinks and toilets, but it doesn't seem to be there directly. It is a smell that seems to permeate both units and the upper inner staircase leading to them.

My body doesn't like it because it reacts with itchy eyes and stuffy nose when I stay there.

Thought it might be the vents, so had a professional come out and clean/sterilize them all throughout the property.

We don't know where the smell is coming from and need to find out so we can eliminate it!

Has this place sat vacant with heat and/or A/C turned off for an extended period?

Yes, the property was vacant for about 4 months.

I had someone mop the floors twice, scrub the walls, and ran a HEPA air filter for 24 hours with windows down. However, the smell came back, and now seems even stronger in stairwell.

Any indication of a leak or mold before you had the cleaning done?

You may want to look into renting a Ozone Generator. That's pretty effective at odor removal in case of smoke damage etc. The units should be unoccupied when its running. And aired out afterwards.

Maybe there is a roof leak and it's leaking between the walls so you can't see it, just smell it. Did you walk all around the rooms and smell the walls, ceilings and floors?

Carpet:confused: