Odd AC related noise coming from behind steering wheel/dash?

I had my AC system fully redone about a year and a half ago, new compressor, hoses, etc. Since then, I noticed there was an occasional slight (but loud) vibration like noise that sounds like it is coming from inside my dash, right behind the steering wheel when I shut my car off and the AC is set to "on".

The noise does not occur if the AC is off. I mentioned this to the mechanic who did my AC system a few days after he did my air system, and he said the oil probably just needed to work its way thru everything and not to worry about it, but if it gets worse to bring it back. Well, that was over a year ago, and I noticed this summer that it has done it almost every single time I use the air when I turn the car off. The warranty on this system is now expired because I waited too long, so I would like to know if anyone knows what this issue may be before I take it in again and have it looked at.

I ignored it the noise originally because my AC still works just fine, and the noise hasn't really gotten any worse, maybe just more frequent, but then again, it is summer, so I'm using my air more than I was in the winter.

Anyone experienced this before? My guess is something just needs to be tightened because it sounds like something is just vibrating from being loose and it slows to a stop when the car is turned off. The noise gradually fades in about a second after the car is turned off, reaches a peak loudness about a half second after starting, then gradually fades back out to silence. It rarely occurs while driving, and it only does so after 1-2 hour road trips with the ac on constantly. I have my AC set to ECO(orange light), not sure if this makes any difference.

I can record a video of what it sounds like if that would help.