Do you have drains in the basement? Have you filled the traps in them with water?

Also have the HVAC system checked out if you find nothing else. There could be a leaky return area.
Do you have carpeting that may have gotten wet?

There is one drain/trap in the basement; it gets regular use (bathroom), so should have water in it regularly.

Thank you for your replies.

There is no moldy carpeting that we are aware of. Additionally, the smell is definitely sewer, not mold (also, people with mold allergies in the house, no allergic reactions correlating to the smell.)

The vents go through the roof (they do not terminate in the attic)

The smell sits heavily in an area that is devoid of air vents but does have a return present, as it is above the a/c unit in the basement.

If it is the HVAC, where could the smell be coming from? The sump pump?

If you have a floor drain, a toilet in the basement won't necessarily keep that floor drain trap filled with water. Check that drain.

I don't think a sump pump pit would smell, but you could put some clorox in the pit and see if it makes a difference.

There is no floor drain that we are aware of; the basement is fully finished with two sewer clean-outs that are nowhere near the HVAC system (across the room, no smell near/above them anywhere in the house).

Our sump pump rarely runs, we've maybe heard it twice in 5 years. Would that make a difference?

Dave, you might be on to something. Hmmm. I do not know where the overflow pan drains to. It's difficult to access, behind the unit and under stairs, but we can check.

Thanks!

Any other ideas pop out to anyone?

Just to clarify, the condensation drain from the air handler is the primary way that moisture is removed from the from the unit. The condensation runs down the cooling coils to a trough and then to the drain line. IF that line becomes plugged for some reason, the water will usually overflow to a pan underneath the unit. That pan in turn will have a drain line. Or in a basement it may not. In my attic units the overflow drain comes out over the deck where it can be seen. I have seen it dripping once and it was because the primary line had become plugged. Note: If you find a problem and have to route the line outside , Make sure you get the water, (all water!)AWAYAWAYAWAY from the house foundation or you will be having different problems.

Once the primary drain blocks with scum and you're on the second pan drain, it's only a matter of time till you have a flood. The best bet is to have a float with a cut-off switch in the pan to prevent this flood. It will shut down the unit before that happens.

I JUST WANTED to say that I live in an upstairs aparment and I have a horrible smell that comes through my vents...like sulfur or gas...it is so often that it makes me feel a little sick...Atleast three times in 24hrs it will blow out very strong through the vents..By tme U can get managment here the smell fades and they say we cant smell it...In my bathroom there is plastic pvc pipe and it sticks out into my shower right behind where my ac is where u change filters..this place has plumbing problems all the time/apt complex. Recently just right outside my section they did some plumbing work...this smell didn't start till afterwards...I am concerned because I don't know what it is and these people overr don't care to inverstigate it...it smells like gas...whatever it is I can't take much more.. I am single with a two yr old boy and I honestly think it is making us sickMy incle called the health dept but no word yet...does anyone have any advice...what is this how does it make us sick...what can I do...

We experience a similar thing. Never did figure it out but it's def HVAC related.