So I’ve had this problem for a while on my 1996 ford ranger 4.0. On hot-ish days, my truck will start to hum randomly. It stops when I accurate, but the second I start coasting it starts back up again. However, what’s weird is that when I turn the ac up, the humming stops, at least for the most part. It’s quite loud, almost as loud as a horn. Anyone know what’s causing this? Just recently my ac has stopped blowing cold air, so I think it’s time to refill it, but the humming started before the cool air stopped working so I don’t think it’s caused by me being low on ac fluid.

 

AC is on or off, there is no other control for it, TEMP **** controls air flow thru Heater core, no connection to AC

Do you mean when you change the fan/blower speed the HUMMM changes?

Could be the fan motor is going out, it has a 2 wire plug on it, disconnect it and see if HUMMM is gone
If so replace fan/blower


Another thing could be AC related, AC works by pressurizing(compressing) a liquid and then releasing that pressure for cooling
The AC Compressor on the engine has a 2 wire connector, unplug it and see if HUMMM is gone
If AC is acting up then the pressure equalization could cause a harmonic vibration which is the HUMMM you hear, long shot