I'm fine in the small chainring, but when I am in the large ring I would say the last 3 cogs on the cassette produce alot of chain noise. I don't believe there is any reason it shouldn't be as quiet as any other gear combo as long as I am not crosschaining.
Could anyone shed some light on what the likely cause would be? All my stuff is new and the RD seems to be adjusted correctly and shifting well.
It may need an adjustment on the rear shift cable tension. Try adjusting the cable at the rear derailleur barrel adjustor in small increments. Clockwise moves the derailleur to the right, counter-clockwise to the left.
You could also check the cassette lockring torque, it needs about 35-40 nm. That's quite a bit. The torque spec is probably shown on the lockring. Loose cogs rattle.
Support the bike off the ground, turn the cranks and try to isolate the noise.