How can I stop my pipes groaning?

Hi,
I've just moved into a new flat and when you flush the loo, the pipes under the floorboards make a very loud groaning sound, with some vibration, which goes on for the period that the cistern is filling up again. The flow of water to the cistern doesn't seem to be affected. I assume this is an air block of some sort? What can I do to get rid of it?
Thanks.