Had to replace the A/C about a month ago (new system is Trane, 4 ton single speed) and yesterday I noticed kind of squeak, or squeal, or metal rub kind of sound coming from the air handler in the attic for a short time when it was starting up. So far I have only noticed this a couple times and both times when the unit hadn't been running for a while like early in the morning, never noticed it otherwise, and it doesn't happen if I turn it off wait a bit, then turn it back on to try and see if it'll make the noise. Any ideas what this issue might be?

I am only guessing that installation hurry didn't get an access door or other internal part "just right" ergo the rub noises ..... and there should be no charge for a new installation inspection/correction visit.

Good backup: even though new
best to start the habit of buying the annual 2/yr maintenance/inspection, discount on parts/etc,

Its weird to me that its not happening all the time and that it didnt start doing it until now.

Have you been up there and identified the location of the noise? I bet it's metal strapping and was cut a hair to long and vibrating against something.

I think Ive determined that its actually causing the grille on the upstairs air return to flex and vibrate