AC noise under throttle

[texdave]

Truck: 76 GMC C25 Suburban w/454 + 3 speed auto + factory air conditioning

When I'm driving and have the air conditioning on (Max or Normal mode, at any Fan speed) I hear what sounds like a short sucking sound every 40 seconds or so. It gets louder as the RPMs go up and I don't believe I hear it at all when at idle. I don't hear that noise when the A/C is off or when I switch the mode to Vent.

Here's a link to a video I took that shows this in action:

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Is this a vacuum leak on a line that goes to the HVAC controls or something else? Appreciate it if you can point me in the right direction.

 

[75gmck25]

The A/C controls run off engine vacuum, and the vacuum reservoir under the hood should keep the controls vacuum relatively constant.

Maybe your vacuum reservoir is not keeping up (leading?) and maintaining vacuum, and the controls cannot hold the vacuum pods in position. This would cause the doors in the HVAC to move back and forth as vacuum rises and falls.