HID lights are very high voltage and they have to generate this from the 12 V system. as such there is a High frequency unit and a capacitance discharge unit.If the screening of the system is breaking down or there is a bad joint the unit produces a low level EMP Electro magnetic pulse. This is picked up in any unscreened wire nearby, The voice coil and magnet of the speaker combined with the trend for low resistance speaker cable produces a circuit similar to an early radio, thus the EMP is picked up by the speaker and can be heard as a "WHUMP" noise, you may also pick up a high frequency crackle too and sometimes nearby VHF transmissions. either replace speaker cables with screened items, or use self adhesive copper tape to cover the cables and then earth this to the vehicle frame.Or alternatively attempt to screen the HID lights and wiring..