Why does my hallway suddenly smell like sour milk and old noodles?

That specific sour-sweet, milky odor is acutally a classic indicator of a dead rodent in a very specific stage of decomposition, or more likely, it's a mold colony that has recently bloomed in the HVAC condensate line. When you describe it as 'nursing home' or 'baby vomit,' you're often smelling butyric acid. This happens when organic matter is breaking down in a damp, enclosed space without much airflow. Since you're on the ground floor and have seen mice near the sliding doors, there's a high probability one crawled into the floor joists or the return air plenum and died near a moisture source.

But before you pay for an exterminator, check the **condensate drain line** for your AC. If that line gets clogged with 'sludge' (a mix of bacteria and algae), it creates a thick, gelatinous mess that smells exactly like sour milk or old noodles. When the AC is running, the fan pulls that scent directly into the ductwork and blasts it thru the vents. The fact that the smell died down when you turned the unit off points directly to the HVAC system being the delivery mechanism. If you can get into the laundry closet, look for a white PVC pipe coming off the furnace or air handler. If there is standing water in the pan underneath it, that’s your culprit. You can try pouring a cup of distilled white vinegar down the drain line to kill the growth. If the smell is truly organic and 'meaty,' though, it’s probably a mouse that got stuck in the blower fan or the transition ducting.

I’d call an HVAC tech first rather than an exterminator. A good tech will pull the blower motor and check the internal cabinets. If an animal died in there, they can remove it and sanitize the coils. If it's just a clogged drain or a dirty evaporator coil, they can clear it right then. An exterminator won't be able to take your AC unit apart to find the source if it's inside the machinery.

While the HVAC theory is technically sound regarding the distribution of butyric odors, I woud urge you to meticulously inspect the threshold and frame of your sliding doors, as the specific 'sweet-sour' notes you mentioned can emanate from moisture-damaged, rotting wood substrates where fungal growth has reached a critical mass. In my experiance with ground-floor units, if the exterior drainage is insufficient and the sliding door track lacks a properly functioning weep hole system, the accumulation of stagnant water can interact with organic debris to create a localized fermentation process that smells remarkably like spoiled milk. Have you noticed any localized swelling in the floorboards or a slight 'give' when you step near the door frame?

Ugh, I've had this exact same thing happen. If your HVAC and doors look okay, check the seal on the actual sliding door track. If mice have been hanging around there like you said, they love to pee in the tracks or behind the trim. Mouse urine has that nasty, sweet ammonia smell that's super similar to what you're describing once it sits in the sun. Since it's right by your vents, the HVAC's probaly just sucking that scent up and moving it around. Try sniffing the actual door frame real close before you drop money on a tech.

I totally agree it's likely being pulled trough the ductwork, but don't overlook the hollow cavities in the sliding door frame itself. If you have vinyl sliders, they have weep holes to drain water, and if organic gunk or a small rodent gets stuck inside the actual frame, it creates a stagnant 'milk' smell that the HVAC intake sucks right up. I had a similar funk and it turned out to be a buildup of wet pollen and dead bugs fermenting inside the bottom rail of the door. Give the frame a good shake or sniff the weep holes directly to see if it's venting from there.

Quick update — I checked the HVAC condensate line like a few of you suggested and it was defintely clogged with some nasty slime. Cleaning that out helped alot, but that lingering sweet-sour scent is still coming from the sliding door frame. If it's a dead mouse stuck inside the actual vinyl frame, how do I get it out without breaking the door?