I am also experiencing a similar issue w/El Capitan update.
IMAC Mid 2011 16gb RAM
OSX 10.11 ( 15A284 )
USB 2.0
POD HD PRO
USB FIRMWARE 1.04
DRIVERS 7.3.8
w/ GARAGE BAND 10.1.0
Constant feedback white noise visible in meters and audio channels. It appears there is an incompatibility issue. If someone has cracked the code please advise. I have filed a ticket.
The POD is fine on own. No effect on my FBV MkII either. XLR/Headphone fine.