A bang could be anything but since you said "when you take off hard or downshift" I don't think it's a brake issue more than I think it's a bushing or cracked swingarm.

Get your bike elevated with the rear wheel off the ground, grab your rear tire and shake it... If the bushing is bad you will have some freeplay in there.
Take your hard bags off too and check for swingarm damage.

OS

 

Could it be the IDS??

 

Not necessarily, that title is BEGGING for comments.

 

Bike should still be under the 2 yr warranty, take that sucker back and make them figure it out.

 

Hold up a bank, hide the money for when you get out, plead guilty and snuggle with some other convicted felon in prison, will help with the bang in the rear!

 

It's all good some how the axle nut came loose. Thanks for the help

 

Cadilacjoe: I just can't tee off on this twice in one day! Really glad that you found that your axle nut had come loose as a result of back rear bang.

Or is it the reverse? I'm so confused!