I have the same exact problem. Had it since day one, sounds like a whistling coming from the right side behind the dash? It only does it when my a/c is off or when it's on re-circulation mode, but only when I'm at speed. Stop, and it goes away. Turn a/c on without re-circulation, and it goes away. That's my only complaint. Sometimes I want to ride with the windows up, radio off with no a/c, and I've got some damn whistling noise bugging the hell outta me.

 

I am not sure but what side is your air intake on? Is it on the passenger side or the other side like others. You may be hearing the motor sucking air when accelerating.

 

It's really not that loud and only lasts a couple of seconds.

Deff not this.

 

That sounds to be exactly it. Doesn't seem like a cause for concern, just strange and a little annoying at times!

 

My 08 ranger had the same sound. Exactly like the A/C cycling on/off even when it wasn't engaged. Haven't noticed it on my 3.7 scab but I know on the ranger when the passenger window was open I'd notice it more.

 

Mine's definitely more noticeable with the windows up.. Having all the air flow through the cab drowns it out a bit.