12v to 5v DC-DC converter hum

Please forgive as I'm just getting back into electronics after a 13 year hiatus. I recently installed a 12v to 5v DC-DC converter from amazon http://www.amazon.com/RioRand-Conve...5W+Power+Supply+for+Car+Vehicle+with+MINI+USB

I installed this dc-dc converter into my 07 HD Road King Classic to power a Miccus Mini-jack RX (bluetooth receiver) using 12v (switched) from my accessory plug under my seat.
http://www.amazon.com/gp/product/B0038MA11U?psc=1&redirect=true&ref_=oh_aui_detailpage_o02_s00

When I power on the device I receive a hum over my speakers. If I run the Miccus off of the internal battery- no hum. I tried various grounds, including the chassis ground bolts and straight to the battery to get rid of the hum. I initially thought it was a difference in the ground potential but obviously that's not the case.

Is there something I can add to the circuit that would get rid of that noise?