Noise from cassette when I'm on large crank sprocket

I've recently bought a Giant Defy 2 which is great but when I change to the larger sprocket there is a noise at the rear like the ratchet kind of noise when you're free wheeling but quiter, it's quiet when in the smaller sprocket but as soon as you change to the larger it makes this noise in every gear.

Does anyone know what causes this?

 

Have you checked the rear derailleur's adjustment?
Is it moving across the cassette smoothly?

 

by "larger sprocket" do you mean the large chainring on the crankset?

 

Yes, the one on the pedals, excuse my terminology, quite new to it and not got all the terms right yet.

 

Is the noise constant when in the large chainring, or does it increase/decrease/go away when using certain cogs at the back?

 

The good news is that MOST seemingly random noises can eventually be isolated & they usually can be eliminated ...

Without seeing your bike, the noise is possibly due to the chain rubbing against the FRONT DERAILLEUR CAGE despite your impression that the noise is coming from the rear of your bike ...

Appropriate adjustment will hopefully eliminate the rubbing (it could be as simple as removing the slack in the front derailleur cable when the chain is on the inner chainring ... with the result being the outer plate being about 1mm further outboard rather than possibly rubbing against the chain).
If necessary, check www.parktool.com and/or YouTube for assistance. BTW. If your bike has a SRAM drivetrain & you encounter difficulties then you may be SOL because Front Derailleur adjustments are limited with SRAM shifters ... but, they can eventually be made if you have the patience (again, check the front derailleur cable tension when the chain is on the inner chainring, first).

 

If the noise is quieter than the freehub racheting noise, and coming from the rear derailleur (RD), it may just be normal. In the big chainring, the RD cage pivots forward under spring tension to shorten the driveline, creating more tension against the pulley wheels on the bottom or non-drive side of the chain, increasing the chain noise created by the chain going round the pulleys. I've found this normal noise is reduced when the chain is clean and well-oiled. In any case, the noise I'm talking about here isn't really noticeable when riding, just when the bike is on the stand and the crank is turned by hand in the big chainring, as when lubing the chain.

If you hear any metallic noise while riding, that's likely due to some mech issues as alfeng has detailed in his post, or perhaps a chain that's been cut too short for the bike which would cause excessive "slack side" tension due to the RD cage to hit the forward limit of travel (not likely on a new Giant).

 

It is consistent throughout the range, doesn't get any better of worse in any gear.

 

It definitely isn't the chain rubbing against the front derailleur cacge as I've looked (though I know what you mean as I thought there was a noise coming from the front of the bike which turned out to be the bottle cage). I've not lubed the chain since I've had it as I assumed it was anyway, I will do that next.

 

Study your drive line well. could be that it's not indexing correctly. I agree with Alf, could be unclean under oiled chain. Especially if you have recently ridden in the rain, or in wet conditions.

 

One other thing you may want to look at is if the cassette is firmly tight on the wheel. You'll have to take the wheel off to have a better look at it. Simply grab the cassette and see if there is any slop in it if you wiggle it on the axle. That would definitely make noise in every gear, and it's not likely you would notice it unless looked at it closely.

The only movement you should have would be the reverse freewheel movement. If there is any slop hopefully it just needs to be tightened a little. Hard to say without seeing it though. If you are unsure, take it to your local wrench have them look at it. Sometimes you can put a spacer on it, but you have to adjust the index if you do.