On my KC, I found some air bleed/vents in the rear corners. Take a look at the plastic pieces that have a small area that looks like an AC / Heating vent. Behind this area is a vent that is open to the outside. The vent is closed, but is easily opened with just a little pressure. It uses rubber flaps to stay closed, but they open very easily. I put some sound deadening material (eDead) from
Elemental Designs over most of the opening and it helped. When I get a chance I will cover the entire vent to see if that cures the problem.
I am not sure what these are for, but I will still cover them if that is where the noise is coming from.