I guess they clear them as usually them (recovery service) poking around checking fuses and connections may introduce further fault codes. It's quite a 'standard' thing for them to carry out a battery reset in these cases. I would guess the buzzing is from a relay, either the ignition relay or starter relay, but whether its a simple faulty relay or 'something else' wiring/PCM fault causing the relay to 'buzz', remains to be seen.