Possibly trapped air, having been inactive during the summer. It's also possible that your radiators may need 'bleeding' with a special key, very cheap, can be picked up in any hardware store.
ours get noisy as silt drifts round the system and ends up at the pump, if you haven't done it for a while, get the system really hot and running then drain it and add new protective chemicals. If a load of crud comes out when you drain it, run for a week ot two and do it again until the water comes out withoout sandy bits in it
On my central heating system there is also a pipe in the airing cupboard, next to the tank, with a bleed nipple on the top. It rises higher than the rest of the pipes. Try bleeding this after the radiators.When bleeding the air out you will need to get the system hot first (turn it on and leave it for half an hour to get the radiators hot). When you come to bleed any part of the system you must turn the heating off. If it is still on when you bleed, the pump can draw more air into the system.