Shouldnt be an air lock but if the noise is when cistern is re-filling I would have a new water inlet valve fitted to the cistern
These can be fairly simple if an in-line valve is in the supply pipe ( quite like a washing machine feed valve )
However if you dont have one you can usually stop the water supply by closing off mains supply stop cock
All depends on the system but a good DIYer or reasonably priced plumber will fix this in 30mins
pc6493
December 2008