The furnace exhausts to the attic? That doesn't sound right. But anyway...
So the problem is worse at night, worse when it rains, and may be coming in through the sub-floor.
My next guess would then be that you have a problem somewhere in a wall or floor, and the odor is traveling through the cavities between the joists until it finds a convenient exit, which may be in the utility room. It could be aggravated by moisture.. as the tenant(s) use the plumbing fixtures throughout the day, moisture is added to the problem. Then it dries out overnight and the odor is reduced.
The quick solution would be to somehow block off the flow of air from under the sub-floor (i.e. put a board between the joists or fill the cavity with insulation and seal all the gaps), but that's just treating the symptom, not the underlying problem.
The brute force option would be to actually do some looking, either by peeling away floor, wall, or ceiling, or drilling holes and checking to see if the odor gets worse as you explore in a particular direction. You could also get an inspection camera (see
https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Daps&field-keywords=boroscope for examples) and feed that through the hole and see what you can see.
More questions:
8. Is the cold air intake for the furnace open to the utility room, or is the cold air ducted in from somewhere else in the unit? Same for combustion air. If the furnace is pulling air from the utility room, the utility room has to be replenished from somewhere, and the floor may be making a small but stinky contribution.
9. You said you've run a smoke test. Have you run a peppermint test?
Not sure what else to tell you.