'14 Prerunner Whistle Noise Related to Throttle/Eng. Vacuum

Ok, I've searched and searched the 2nd Gen forum and could not find anything like what I'm hearing. If I missed it please be gentle.

The ride: 2014 Prerunner Access Cab with 2.7 and auto trans. - all stock.

The noise: An interior whistle noise coming from what sounds like the rear part of the access cab. This is not a wind noise; I have heard it with truck stopped and transmission in Park. It is definitely related to throttle position and/or engine vacuum. Very brief, just 2 or 3 seconds, comes and goes depending on throttle position/vacuum, especially while transmission is coming up through the gears.

I don't remember hearing this whistle in the warmer months but I could have had the radio up too loud. Wanted to add that because there might be a possibility that cold weather is a factor. The more I hear it the more it bugs me and I would love to put a stop to it. Has anyone else experienced this or know what is going on? Any and all help appreciated!

 

Thanks Rock Lobster but definitely not the smog pump. First time I heard that smog pump sound it did freak me out! This is something else though. I tried to record it this morning with my phone but it didn't come out - will try again. The sound that's bugging me is like a light whistle that sounds like it's coming from the rear of the cab (AC) but sometimes sound direction is misleading. When the whistle sound starts I can affect it by changing throttle position. Thanks again for your input.

 

Inspect your intake system from air filter box to the throttle body.
How about the fan belts?

 

Thanks, I have looked over the intake system but didn't see anything suspicious, and it's not a belt noise. I caught a little bit of it today at lunch on my phone but can't upload file type .m4a from iPhone recorder.

This thread http://www.tacomaworld.com/forum/2nd-gen-tacomas/337750-2014-tacoma-whistle-noise-air-vent.html mentions a "fresh air outlet (behind the back seat assy)". Does anyone know if this outlet might be connected to manifold vacuum somehow? It sounds like it's coming from the rear of cab but my ears might be playing tricks on me.