Is the head unit original or aftermarket ? If the latter, then the installer has probably bodged the wiring.
Original High Line HU should have 10-way C224B from the radio, with a pair of wires to each amplifier. Check the continuity from this to each door amplifier. They are a balanced pair, so a break in either will cause buzz, but the audio might still work.
Also check Pin-6 on each door amplifier is properly grounded. All four amps have different ground points !!