I have a compact digicam and sometimes iyou can really notice the grain/noise especially in the sky area. I don’t know why does this happen, it’s a 10mpx sensor with image stabilization and I’m usually using low ISO when shooting on a daylight. As for shutter speed, it’s almost always set to default.
My old CCD-based DSLR would start showing noticeable noise at ISO 800, and a smaller sensor could be worse. Sunrise/sunset can be deceptively dark; I would check the files’ EXIF info to see what the ISO was. If you’re on Windows, the file property should have a Details tab that lists the major EXIF data. You could take a couple pics, some with noticeable noise, some without, and see if there is a correlation with ISO values, and see if there is an ISO value threshold.