Id say Cap on the steering pump.. mine did that on my 5.2L thought it was a flat vac leak but it was just the power steering cap sucking vaccum through its seal for what ever reason..

 

I have the same issue. Haven't had a chance to look into it. Same sound. Doesn't always happen. Sounds like it comes from under front dash. I'm thinking a hose is leaking a little somewhere but doesn't seem to be the heater/ac blower. It is annoying but all I have to do is change the speed a little or I guess rpm's. Truck runs good. New truck to me.