Smell from HVAC system

That is the configuration of my HVAC system that was done by the person installing the new furnace. The black pipe is the drain pipe for the house.

https://imgur.com/a/CpLsK

Over last few months I have noticed very faint smell of rotten eggs when the furnace is not on. I would give it 2/10 and I have to put my nose in the vent to smell it. It also happens at the top level of the house and not really at the lower levels. I have tried to smell the top of the black pipe thinking it might be coming from there but nothing there as well as furnace room in general.

Any ideas what might be the cause ?