Hard Shifting / Ghost Shifting
I find most full suspension bikes have a tendency to Ghost shift ( Shift on their own). This usually is due to cable length issues around the suspension pivot. This can be fixed with revised cable or complete housing front to rear.
Hard shifting, alot of banging or clanging during shifts, has to do with rider technique, derailleur adjustment, and the design, quality or wear of the rear cassette.
A downshift to a bigger cassette gear during a high load climb can make a lot of noise!
Also an upshift to one of the very small cogs at high loads, say while hammering out on a flat section, can also be loud.
The basic rear derailleur adjustment to check is to shift to the middle crankset ring and the middle cassette gear. Looking from the rear of the bike, are the derailleur jockey wheel straight below the middle cog? ( I mean straight below) otherwise adjustment is needed.
Here a basic article on derailleurs.
http://www.sheldonbrown.com/derailer-adjustment.html
Ken S.