Pacel Shelf speaker issues

Heya im relatively new to the whole car thing as i recently just passed. I am having issues with the electronics in my 2003 renault clio dynamique. Essentially i bought this car second hand and with it came 2 6 X 9 parcel shelf speakers that were installed with the orginal renault headunit. I replaced this headunit and to start with everything worked fine but the main issue im having is that now every time the music is played above 20 volume which isn't very loud the parcel shelf speakers start to make a crackling noise. I have had to use the fader to make most of the noise come from the front but i really need help i want to get my speakers to work. I've been told by mates that it could be to do with the preout? i know that a preout is used for amps but im not really sure how this could interfer with the speakers. Anyways please if any could help me out it would be much appreciated

Thanks:p