Q: Idle fluctuates and then stalls.

Hello!

My car's battery was dying toward the end of last year, so I had the battery replaced at a mechanic's shop. Then, less than a month later, the battery completely died again. I bought a new battery, set up an appointment through YourMechanic, and someone came out to replace my alternator and battery and do a visual inspection of everything else, and my car ran normally for about a month of very infrequent driving until this weekend. Now, all of the sudden, the car has an extremely low, rough idle when stopped, and there's a loud screeching noise coming from the engine area that's loudest when idling and a bit quieter when gas is applied. The visual inspection said that most of my car (including engine, belts, etc) was "excellent" so I'm not sure what the problem could be. Any help would be appreciated!

My car has 66000 miles.
My car has an automatic transmission.