Ok lets clear a couple things.
I was right in what i said that depending on the version will vary from 500mA to 900mA.
USB power is quite clean compared to a cheap power adaptor kind we find on the shops built in china ( ill give u an example- same as you built something, all sounded top, with a wall adapter, noise comes).
As said before you need a good decoupling, the cleanest psu u can get, and preferably a metal enclosure to avoid emf interference ).
Look for hot to filter hum and ground noise, as theres los of tutorials on how to, and decoupling of course !