i have done several long exposure star trail shots and while long exposure noise is pretty much inevitable and I agree with all of the info that has been offered I am also noting that you say nothing about in camera noise reduction. I always run my camera (Nikon D90) with the long exposure noise reduction on. The up side is that it does clean up the image noticeably (I have a friend with a much higher end camera and the long exposure noise reduction on his camera does a fantastic job), the down side is that for every minute the shutter is open is another minute the camera needs at the end of the shot to run the noise reduction eg:45 min exposure is really 1hr30mins by the time the camera has done noise reduction (which can be a big ask for some batteries).
I have always waited until full dark midnight-ish with no moon to get nice clean shots with a minimum of non-star light.
A friend of mine who is a light trail fanatic has a group of settings that he uses as a formula for his exposures and he worked these out after taking a heap of shots using different settings and finding the combo that works.
Hope this helps to add to your experience
I reckon just keep at it and you'll find which settings work best for you.
Cheers,
Aug