The strange metal noise is most likely just the electric blowback system. If you would rather it not do this I can tell you how to disable it.
From personal experience I would suggest;
Madbull Tightbore barrel
Prowin hop unit
Madbull Red hop rubber
Any non-linear spring
SHS High speed motor
Turnigy Nanotech 7.4v lipo
Also good quality BBs make a difference (Blasters)