At a bit of a loss. Noticed a mildew like smell only in the main bedroom a couple days ago. Checked all the drains in the attached bath, but they seem fine. No signs of any water damage or mold. Went in the attic directly above and can also smell it, but just right in that area, other parts of the attic are fine. Also no water damage, mold, or signs of animals in the attic. Moved some insulation around in the area and nothing.

Tried turning on the AC and it got better. After work today when the AC wasn't running, smell was back. Tried running fan only and the smell remained. Took off vent covers and checked return, all clean. Turned AC back on and it got better almost immediately again.

If it was the evaporator coil, I would think the whole downstairs on the same system would have the odor, but it's localized to just the one room. Maybe mildew in the branch feeding that room? But the air coming from the vents, even on fan only, didn't smell.

Other recent change was we got new windows installed in just that room on Saturday, but can't see how that would cause it. Any advice or ideas would be greatly appreciated!

What changed? The windows did. Focus your examination there.

The windows all look fine. I did think maybe them being open for the afternoon during replacement let some humidity in, but enough to do this seems like a long shot. But maybe there is something about the replacement process.

I did crawl around the attic again and the 2 vents to that room are on a separate branch. Off the main it splits once, with one branch then splitting to go to the bathroom and closet for the room, and the other branch splitting into the 2 vents for the room. Bathroom and closet are fine, no odor, so it if was something in the duct it's isolated to a relatively short section. Thinking of temporarily blocking those 2 with the AC off to see if the smell dissipates.

Running the ac can reduce humidity and obviously increases airflow. It may not be the cause, but a short term solution when Running.

I had a similar issue. I tried running it down a dozen times. Purchased a dehumidifier and the problem went away. So I purchased one for every bathroom.