Sony sa-w442 volume issue

The input jacks (RCA type) for my Sony SA-W442 Active Subwoofer are acting up causing the sound/volume to  intermittently go either way down or completely out. A tap with my foot to the front of the unit fixes it, but it eventually does the same thing again after a while (I assume from vibrations). Can anyone tell me what exactly is causing this?? Thanks in advance