High pitched mechanical whistle while coasting in gear

Hi all,
I'm new here and I hope this is in the right place. My 2008 taco (v6 4x4 sr5 112k mi) is making a faint high pitched whistle while coasting in gear. It kind of sounds like cicadas. The sound is most noticeable at 30-40 mph and stops immediately when the clutch is depressed or when accelerating. I can't tell where it is coming from however the other clues point to drivetrain.
Any thoughts on what this might be? I have to deliver some materials to nyc (3 hrs away) next week and I'm a little concerned that he this might become a larger problem.

 

What are your RPM's at? Mine makes a high pitch whine around 2k until I shift or push the clutch in. It's done it since new. The dealer says it's fine but I def know when I am not really on the gas.

 

I haven't paid much attention to the RPMs. I'll check it out when I go out later today.

 

My truck does the same. No idea if it's normal or not though.

 

So I just left my dealer I had them check fluid levels on the transmission they said it was fine but they topped it off, in the 10 miles or so I have driven it seems to have lessened a bit. Might want to just replace the fluid.

 

mine makes a diesel turbo like sound when accelerating

 

Mine has always done this in certain scenarios: coasting downhill around 65 mph there's a whistling, going about 40 mph in 4th gear there is a pretty noticeable "Ooooooo" noise.

When I still was under warranty I had the dealer look at it. They heard the noise, but found nothing wrong.

50,000 miles and a fluid change (tranny and rear diff) have passed and nothing has changed, so I ignore it now.

 

My '14 howls since new up to and including 60 mph on deceleration. Pisses me off!