Sound cards sometimes can bring in ground loop problems.
I installed a Creative Sound Blaster bla bla pci card. My front amp was ok, my sub amp was ok, my rear amp had a little buzzzzz.
I bought a $17 Radio Shack ground loop isolator (gli). Buzz went bye-bye. Sound quality not diminished one bit.
Happy ending for me; maybe it could work for you.
Best of luck and blessings.