I added this > http://www.homedepot.com/p/4-in-x-25-ft-Insulated-Flexible-Duct-R6-Silver-Jacket-F6IFD4X300/100396934 but removed the plastic liner and used the stock foil ducting. Then used aluminum tape to seal the ends.
This DEFINITELY takes the ‘shrill’ sound out of the exhaust.
Thanks to @takitus for pointing out that covering it dampens the sound a bit.