Cooler Weather...New Whistling Sound

I'm new here and I searched this forum as far back as I could but didn't find any clues to my problem.
I have a 2007 Tacoma SR5 Crew Cab 4wd and I have a new sound under the hood since it's gotten cooler out. When cruising at highway speeds in 6th gear with the engine revving around 2200rpm I starting hearing an intermittent 'whistling' sound. At first I thought I might have an idler pulley going out but this morning I started it up when it was cold (45F) and had someone else hold the engine rpm at around 2200rpm and could clearly hear it. I now don't believe it's an idler pulley because with the truck stopped and my ear closer to the engine it sounds more like some kind of a vacuum/air valve cycling or venting near the top of the engine. I took off the upper engine cover and it seemed like I could hear it a bit more clearly, however, I was unable to pinpoint the exactly place it was coming from. The sound comes and goes about every 20 seconds or so and lasts up to 30 seconds until it stops. The funny thing is that as the engine started to get warmed up the sound happened less frequently and then pretty much stopped. When the truck is moving I hear the sound regularly likely due to a constant flow of air moving around the engine.
I really don't want to take this thing to the dealership. Any ideas??