I appreciate the comments and feedback
I am going to head up tomorrow and check for myself. I’m taking several tools from a black light flashlight to moisture meter to thermal camera to check everything out and smell the smell for myself to see if I can figure it out.
The vent stack for the water heater is a straight shot vertical up, and is a solid piece going to the roof, same for the furnace. This used to be our primary and when it was I sealed all the crevices for critters to get in. When I walked the property there was no smell, no sign of critters anywhere… never had (besides squirrels one time) issues with mice or rats. Where tenant says he thinks the smell is the dead space behind the filters that feeds the AC unit air, but doesn’t smell it in the rest of the house.
Tenant hasn’t said wether it is intermittent or not, just that it smells real bad, but only in this small area of the house.
I’ll follow up after my visit.