just thought. this started happening after I removed the rear 6x9's that came with the car, and I've left the two chunky speaker cables lying loose on the metal bootfloor. they may even be touching. could something as simple as this be causing the interference to my 2 fronts?!
I would go check now, but I'm in my glasses, trampy jumper and wheelchair pants eating biscuits so dont want to be seen by my neighbours or the general public.