noise is a problem with Canon and long exposures....the longer the exposure the worse, generally, the noise is....i try to avoid a large sections of flat ,no detail, sections, i.e. a flat blue sky....
try limiting your long exposures to less than a couple of seconds (which is usually plenty long enough to get the milky look)..
i've got a canon G5 and i do get noise but can usually clean it up with filters etc.