Hi,

We have just noticed that when the gas comes on, it sounds like running water through the pipes.

Do the lines need to be bled?

Shut the gas off at the meter. Look for a 6" long nipple pointing down from a tee on the gas line. If you have one then you have a "drip loop" installed. This is there to collect condensate before it gets to your pilot lite. Remove the cap and drain the line. ( It should have been installed with a little back pitch) Good luck, Tom