Yes this noise does it while in gear, stationary, when a constant small amount of gas is applied when im on the brake. So im pretty sure its not the differential, speedometer, or transmission.

 

Have someone help you by sitting in the truck while in drive and press the brake and rev the engine a little. Have the hood up and stand next to it (on the side, not the front!!) to see if you can locate the sound.

Use someone you trust, so they don't run you over!

 

In addition, I wouldn't completely rule out the transmission (or some part of it) just yet...

 

possible a/c compressor clutch cycling, it will click in and out with the presure.

 

Thats definitely not it...sorry. I know what that sounds like. That was just replaced so that is good to go.

 

Did you identify exactly (or close to) where the noise is coming from?