Your camera should have a LONG EXPOSURE NOISE REDUCTION setting. if it is enabled, the camera will take a 2nd 30 second shot with the shutter closed, and it will then subtract that noise from the first pic. This can also be done manually by putting the lens cap on and doing a long exposure and then using software to do the subtraction. The first way is easier, but not always practical. YT search how to do both for details for your specific camera.
Pixieset.