when you turn the ac on it makes a loud humming noise
My car has 150000 miles.
My car has a manual transmission.
The noise is most likely coming from the A/C blower motor in the dashboard. The blower may have excessive build up of dust collecting on the fins or has sucked something into the blower causing an imbalance to it. I would remove and inspect the blower fins in the evaporator case, located behind the glove box.
If you’d like to get this fixed or simply want a second opinion on this, consider YourMechanic, as a certified technician can diagnose the issue with your A/C and help you fix it accordingly. Good luck.