A/C Blend door noise

[scooby2b2]

Hello, I am new to the forum and sure yall have had this question a dozen times but while going down the freeway with A/C on max, The blend doors make poping noise and air volume goes down like yuo swithched from max/recirculate air to normal outside air then a few min. later back to max. This goes on all the time and less noticable the slower you go but you still get the pop noise. any hints from anyone?

[04sierracrewcab]

my work truck has this problem ac blows less with speed/ acceleration i believe its a vacume leak of some sort

[scooby2b2]

I thought these were elctronically controled, not by vacume like the older80's style ones were. I could be wrong though, I will look further into it this weekend and let yall know.:glasses:

[04sierracrewcab]

I thought these were elctronically controled, not by vacume like the older80's style ones were. I could be wrong though, I will look further into it this weekend and let yall know.:glasses:
the work truck is an 84 ...
but the newer ones are electronically controled :neutral: