Noise can be a problem, even at ISO 400. That's about as high as I normally shoot unless I'm forced to go higher.
There is a free version of Noiseware available that may help. Here's a link if you'd like to try it:
Your spot on noise is much lower in lighter colours and far higher (and therefore more noticeable) in darker colours.
This is all to do with how the sensor in the camera works.
Noise reduction software can make a difference but be careful it can remove detail. I try to use it only on big open areas with little colour changes.
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