If the noise is coming from under the floor or within the walls, away from the boiler, then the most likely cause will be that pipes are being restricted from expanding and contracting when the system is in use. Radiators may also be heard to creak in their brackets.
Pipes expand when hot and contract as they cool, e.g - each 1-metre length of copper pipe-run will increase by 1 millimeter over a 60°C temperature gradient.
To remedy this you need to lift the floorboards or break into the wall and make provision for movement. Pipes that run parallel to joists should be secured with clips that allow movement whereas pipes that run across joists should sit in large enough notches to accommodate felt lagging to cushion against rubbing.
Alternatively you might consider adjusting your programmer so that your system comes up to temperature (and cools) at times during the day when the creaking noises will be less obtrusive.
Humming noises usually emanate from the circulator pump. Vibration noises of this type can be caused by an ineffectual mounting bracket or wear to the impellor shaft/bearing.
If your whole heating is noisy you can try adding a chemical such as Fernox or Sentinel Boiler Noise silencer.
