Sewer smell coming from air conditioner drain.

[Benson242]

I live in an apartment and maintenance can't figure out the issue.

What happens is a sewer smell comes from the AC drain and spreads throughout the apartment when the AC is on. The cause isn't the vents since they were just cleaned. I've poured water down this drain in case the p-trap was dry. I've also poured bleach and pipe cleaner down it but that only eliminates the smell for a few hours.

The smell comes and goes. What I've noticed is when my neighbors are using their shower the smell comes out. And when they stop showering there's immediately a thump sound like something just closed in the pipes. What is happening?

 

[WorthFlorida]

In an apartment the A/C condensation drains into the sewer pipes of the building. You were correct to pour water down the drain to fill up a P trap if there is one further down the line. a few things that might be going on.
If you do have an A/C drain trap (Google "A/C drain trap" for pictures) they do not have much of a dip and usually only a 3/4" PVC pipe. These a/c drains are not vented and they are connected directly to the waste line. Now that the vent pipes have been cleaned, every ones drain is working 100% and the water is running down the drain faster than before, hence this rush of water is pulling air it could be sucking the water out of the a/c trap.
High air back pressure in the air handler is push the water out of the a/c drain (with or with a drain trap) since water will drain easier after the air vents have been clean.
If there is no a/c drain trap, add one. Easy easy to do using 45 degree fittings if you can find a preformed one.

Two things about the thump. lose pipes in the wall or some one replaced a Moen Shower cartridge with a DANCO brand. I posted this a few years ago on this forum. When the bathroom faucet was shut off, the cartridge from shower faucet would cause a as if someone tap the pipe. Repacling it with a Moen brand took care of the problem.
https://terrylove.com/forums/index.php?threads/water-hammer-knock-danco.32514/