Ok, so today I was driving on the highway and I thought I would try my A/C (have checked it before, but never used it much) And I have it just on regular where it would come out at the face level vents, I was cruising at about 65 and any time I would go to speed up, or go up a hill, the truck shift, as usual, but I would hear a pop and the air would stop and start coming out of the defroster vents. This would happen anytime the truck shifted to either speed up or get up a hill.
Then, I turned off the A/C and just went to face vents on cold and it did the same thing. But, if I turn the heat on, it's fine and stays where it should.
I never noticed this before because I haven't been driving much where it was hot, or warmer out (I got my licence last fall) Any idea why it's doing that?

 

Do a search on "wild vents". Haven't had the issue myself but it's a common one.

 

This means that you have a vacuum leak.

 

What's the best way to find and fix problem?

 

Its not necessarily a leak, theres a little canister type thing that goes in-line with the vaccum line that keeps it from doing that. Mine and my dads are the same way. Not exactly sure what its called

 

Vacuum check valve. There is a thread about Wild Vents in the DIY section that I think has part numbers for them.

 

This is the first place I'd check if you haven't messed with or broken any vacuum lines lately. However, there are quite a few other possibilities of a cracked or dry rotted line somewhere.

 

I found that my check valves were stuck in the closed position...most likely from lack of use while the vehicle sat for awhile...i pulled them out, blew through them a few times, put them back in and works well. Ive read done people blast the check valves with wd40...what do you have to lose? If either doesn't fix them, youre buying replacements anyways. But first read through the "wild vents" threads...it's how I fixed mine.

 

Can someone provide a link the the "wild vents" thread I can't find it. I did a search but I keep getting too many results to find any answers

 

I can't find the one I had in mind either.... no idea what happened to it. Have a look at some of these threads though. (I hope that works.)