How do I find aleak in my central heating system?

After paying people to find out why I'm still having to bleed my radiators nearly every day, the prpblem hasn't been solved! Now I'm lead to believe that there is a leak in the system. Non of the radiators are leaking. Help I've no idea where to go next Recently had new thermo' valves fitted and one gives off a high pitched whine when on a certain number, would this have anything to do with it? I can't the plumber to come back although he made a pigs ear of one valve. He fitted it to close to the wall and I find it impossible turn. Thanks for reading. Iris