why so much noise?

Hi,

Last week I had been to a beautiful place and as usual did a lot of photography. Got many early morning, late evening shots. Now when I am checking them on PC, I found that most of them are too noisy. I had not seen this problem before. I can see this problem prominently on blue colored areas. Few warm toned photos are good. But all photos where blue sky or water is visible, it seems to be very noisy. I have cleaned up the sensor just recently from a Canon shop.
Please see the attached 100% crop

what were the settings on these pics? Was your ISO too high?

Thanks for quick reply.
I should have mentioned before.
1st photo 400, 2nd 800.
But this is not the first time I am using these ISO settings.

Although I dont have an answer for you to me that looks like lot of noise for ISO 400 and 800, I shot several shots the other day at ISO 1600, such as the one attached without that much noise

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I have read that underexposed pictures tend to show more noise. Is that true?

Yes, underexposed images will show lots of noise regardless of ISO.

Yeah then may be this is my problem this time.
Those photos are underexposed a bit.
and the other photos include a snow covered mountain shoot in dawn light and series of photos where tip of the mountain looking red during sunrise.

I was expecting to get the blueness of dawn, and later the red/ golden color of mountain. During my trial shot it showed the well exposed scene, but it was loosing the mood of color. So I thought of making it underexposed.

Few bird photos which are taken before sunset are well exposed and thus no noise.. obviously they show the warm tone of evening. And I thought the other way.

Now the question is, if I want to shoot during dawn time and show the blueness of things or specifically the red tips of mountains how would I do it? The proper exposure was making it as white mountains instead of golden red.

the shot attached here is what I wanted. but I had considerable noise. I have tried to remove it from blue part as it was plain blue.