I recently had my 700r4 rebuilt, it has a reverse pattern full manual valvebody. It drives well and shifts firm as it should, although I noticed a howl coming from the transmission. It is only in 3rd gear, loudest under light throttle application. I plan on pulling the trans back out so it can be gone through but I was wondering if anyone had any ideas beforehand. I attached a video of the noise in question.

 

Don't see a video. You're sure it's coming from your transmission and not from the rearend (speed related to 3rd gear)?

 

I thought rear end initially, but I was driving the car about a month ago before trans went out and rear end was perfectly fine.

 

It would most likely be the Front Planet.
Have fun!