The Central heating system has many components and a very complex structure. Taking all this into consideration you will find that in time it will develop problems that cause strange rattling noises.
Panel Noises
The heating unit uses devices such as the fan, the ductwork and the compressor which are covered in panels. In time the screws that hold the panels can loosen causing the vibration of the devices to affect the panels that will make a rattling noise. In this case you need to refasten the screws.
Motor Noises
The motor of the heating system wears out over time causing grime and dust to be collected inside fans. The motor has belts that can get stretched and displaced, eventually causing the motor to make a lot of noise. This problem is eliminated by replacing any faulty parts inside the motor.
Sputtering Fuel
This is the less wanted situation. If the heating unit works with oil, then it might spill over the ignition compartment. Fire will make the oil burn and while it burns it will make a rattling sound. For heaters that use gas you can identify problems with the gas regulator. Small explosions might get created because of the puffs and gas buildups. You have to realize the importance of these problems and take actions immediately.
