Pipes with a pulse!

[phillmyglass]

Hi, I am on with fitting a new bathroom so have drained the mains down a couple of times in the last few days. When the water has not been run for a while the pipes develop a regular pulse tapping noise. If I turn the stop tap off it stops immediately, back on it starts immediately. If I run some water off it takes a while to start again , but always dose. Is iit an air lock?will it just work its way out?
Thanks for any ideas or advice.