what do you mean by 3 pin mains input. yeah all my house have a 3 input plug. but my laptop charger (present) only have 2 input
OK! Now we know! Forget everything about earth loops. It looks as though you have a "dirty" power unit. Is it an actual Sony PSU? if so I would give them a hard time since...
1) The PSU should be clean.
2) The laptop should be filtered against a reasonable amount of DC borne crap!
Dave.