Air bubbles can occur as a result of the hot water moving inside the pipes, and they will make little clicking noises. They can be dealt with fairly easily by waiting for the system to cool down so the air bubbles rise to the top, then bleeding the radiators. You may have to do this more than once to completely get rid of all the bubbles.
A greater amount of air in the system will cause a gurgling noise. Again, bleeding the radiators should sort the problem.
If there is a lot of trapped air in the central heating system, then there may be a problem with the feed and expansion (F&E) tank, which is usually in the loft. The F&E tank works in a similar way to a toilet cistern.
If the tank is dry, it is probably a result of the ball valve becoming stuck in the lifted position. Wriggle the arm up and down until it frees and the water flows back into the tank. If that doesn’t happen, the ball valve will probably need replacing.