hi, i think you will find that there is a wire loose or a bad earth connection. this popping through the speakers is caused by a potential being held up and then being allowed to flow. the resulting current flow is usually vented through your speakers (the last outlet if you like) it is like putting your foot on a hose pipe, then releasing...you know the effect. i would suspect the earth side personally, but it could be the live side. you may have disturbed something other the the wires you removed, something in the dash, or the earth return on the chassis inside the dash...

i would follow the cables around as far as poss and see, you may see something obious, costs nothing either...

regards
kenton