wierd noise from the rear...

Put a lot of miles on the truck yesterday pulling the boat. I noticed today a wierd sound coming from the back end.

It seems to be making a chit, chit, chit noise as I accelerate(I hope I dont get banned for this). As soon as I get off the gas it stops. It only makes the sound once I get up to 15. And as I gain rpms it gets faster.

I put my truck on stands today though and it wouldnt make the noise, so I assume it has to be under load.

What would ya'll start looking at?