This buzzing feedback is single coil hum, the middle p/u on Strat is normally RWRP and in positons 2&4 this causes the Hum to be Bucked because you are using one of the other p/u's in combination with the middle p/u. Three routes you can go, Improve shielding, which will be an improvement but not a complete fix, Noiseless Single coils, or single coil sized humbuckers.

BTW, RWRP= Reverse wound, Reverse Pole