Speaker buzzing

[Dayvo]

Ok,ever since installing an amp and replacing all speakers a few years ago I have this persistent problem of the right rear door speaker fading out and then starts to buzz which is consistent with engine revs. I guess it is an earth problem but could I get a bit of advice as to where to start looking please. The music is still only just audible from that speaker when it fades out and when i turn the volume up the speaker return to normal . Thanks in advance guys

 

[degeneration]

Could be a ground loop or bad connection