A/C causing exhaust sound change / drone

[1972BlueC20]

I have a 72 C20 with aftermarket A/C, it was added a long time ago. Ive noticed that when I run the AC it drastically changes the exhaust note. It gives it a major drone. Is this normal?

If so, why? Is it just because the extra strain its putting on the engine?

[brown7373]

Belt configurations are different depending which accessories you vehicle has (*powers steering, A/C, A.I. R etc.). The various belt configurations are supposed to balance the stresses put on the different pulleys. If A/C is added it may be stressing one or more of the pulleys, and it is worse when the A/C is turned on, as the resistance is increased. I'd check to make sure all the belts are tightened correctly.

It could also be making a bracket vibrate. I think my vent pipe to the air cleaner hums some.

[Howitzer]

Is this when the blower fan is on high(er) setting? Sometimes when you have multiple spinning things going on, they each create their own frequency, and if that is out of sync, you may "perceive" it as a drone, when in fact you have two competing spinning objects of similar frequency, but not exactly the same. Does it make this sound when you're outside the truck and someone else drives by in it? This of course is all a wild guess!

(i relate this because an airplane i used to fly usually had the propellers synced, and if you turned the blower fan on high, it sounded like they were no longer spinning at the same RPM, even though they were) I know, it's a bit of a long shot to relate it to a truck!