Window A/C makes loud vibrating noise, but only in 90+ degree weather
I have three window A/C units in my apartment. All are the same basic model (Frigidaire Quiet Temp), but two are 6,000 BTU and the other is 8,000 BTU.
They all work fine and are generally quiet. But in very hot weather, like 90+ degrees, the 8,000 model makes a loud buzzing/vibrating sound when it’s running. (Only on “cool” mode, not on fan.) It's always done this since we bought it in 2018.
Normally, I’d think it was a problem with the installation, but then why would it only happen on super-hot days and not on regular-hot days?
Or maybe it’s that the A/C is working harder in hotter weather, but the buzzing sound begins immediately when I turn it on. Would it start working harder so quickly? And why don’t the smaller, less powerful units make the same noise?
Any ideas what’s going on? The noise is driving me bonkers.
Thank you.