Quafie Whine Question

After installing my VAC built Quafie last weekend, I have a slight high pitched whine when under light decel (engine braking in 6th at 70mph) or slight accel (cruise control on the interstate). It is not very loud, you can't hear unless the radio is off. The noise reminds me of the whine you hear when driving in reverse on most gear boxes.

Any ideas on what might cause this? Diff needing to be broken in? Improper build? Defiv lockdown NVH? I also have stock subframe bushings.