Yesterday we noticed a foul oder coming from our vents throughout our house. It smelled even worse in our basement. I figured it was the drain because where the air conditioner drains there was a puddle of slimy water surrounding the drain. I cleaned up the puddle and then scrubbed that entire room, followed by pouring vinegar and baking soda down the drain. I then turned on the central air thinking that it would be fine... only to notice that the smell was still there. So I then changed the filter on the unit and the smell seemed to go away only to come back a couple hours later. If it was the drain wouldn't my basement smell all of the time?

The A/C drain should have a trap iinstlled in it to prevent the drain line from drawing the smell from the drain back into the HVAC system and causing a odor.

The return air on a HVAC system is working under negative pressure which means that any odor source close to the return air duct that is not properly sealed creates a possibility that the odor will be drawn into the duct area and blown out into the home through the duct system. Also a small dead animal will also cause a nasty smell in the home.

All forms of smelly nasty bacteria and junk build up in your condensate trap, condensate pump, catch pan and drain lines that smell just like that. When installed and working properly you should not get that odor. So you need to have that checked out to be sure everything is in order. You may also need a good coil cleaning.