edit background 'hiss' on radio broadcast in GarageBand ?

I have set up my Denon tuner next to my desktop Mac, and connected the two either directly with a Belkin RCA Audio Cable into the 'audio in' port, or via an iMic with a USB connection.

I have linked the tuner to my external aerial for reception, but can only hear it through the Mac speakers because it is part of my 'seperates' sound system, and doesn't have any dedicated volume control or volume output

I have done test recordings using both USB and 'audio in' port options, but in both there is significant background 'hiss' (which can also be heard when just listening to a broadcast through the Mac speakers)

Is there a way of editing the background 'hiss' in any recordings in GarageBand ?

Or of preventing the 'hiss' coming through in the first place ?