A few common causes are;
1) Garbage disposal- the underside of the rubber gets full up smelly gunk.
2) A cracked Air handler drain pan leaking condensate into the bottom return air duct below.
3) A dried up P-trap from an unused bathroom or laundry tub.
4) A malfunctioning AVP trap under a sink.
I would check these things first. Hope this helps!
Frank
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Florida licensed Home Inspector #HI688. (352) 250-7818
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