Please help identify this sound

Hey everyone, this is my first post. As with most people, it's because I'm in need of some assistance that I haven't been able to find the answer to. My skills are limited to the simple oil change and this is my first truck, so please lower your expectations of me.
So my issue is that there's a buzz/hiss/whine sound after starting up & warming up the truck. It's one quick sound occurs once the RPMs lower. Then, also at shut-off, it makes an even louder buzzing/whirring/vaccum sound. This occurs immediately at shut-off. I'm at 35k, nothing is cracked or deteriorated, or any holes to be found. If you guys know of a way to upload audio (mp3), please let me know. Thanks in advance.

 

Secondary Air Injection - Emissions Pump. = Normal

 

Alright, thank you sir!
That is a relief.