You have taken the correct path thus far in your trouble shooting.
All the audio electronics should be wired to the 'Common' battery post (and not the battery bank post if that post is battery direct) if it is a conventional dual battery switch. If the computer is accessing power from a different battery bank then you have introduced a supply loop that is typically noise inducing. If the computer induces noise then the computer needs to access the the identical B+ point and the identical ground point as 100% of the audio gear including the source gear (HU & EQ). You can canabalize and standard RCA ground loop isolator for one of the small EI transformers contained inside. The transformer can be used to break the DC connection on either the DIN cable shield ground, another internal DIN cable ground wire or both.