Hi,
Yes the noise seemed to come from the right as you sit on the bike.
Does your engine turn over ? or does it just make the rattling noise?
I think that relays can arc across the contacts and weld themselves open (or closed) if the battery voltage is too low, so when I heard the noise I did not keep the button pressed very long
.
Also had no error codes and the lights all came on with the key just not enough juice to turn over the engine.
Mine was on an Optimate and started the bike no problem over the winter. Kept outside in minus temps and always started.
It seemed to die really quickly. The new battery spins the engine over a lot faster now.
You could just have unplugged something when fitting the PC3, but I don't know what would still light up the dash and not start the engine with the relay chattering.
Hope you get it sorted.