Banging from pipes when heating comes on?

[cybil2403]

When my heating comes on for the first 20 mins or so the pipes are making a banging/knocking noise? Could it be my hot water is set too high/hot or theres a blockage?

[Hawkwalk]

Probably air in the system. Try bleeding the radiators, this should cure it.

[buildersmate]

More likely the copper pipes expanding (slightly) in response to being heated suddenly by hot water inside them. This is why it happens on start up but not on cool down - it reverse process happens more gradually.
The expanding pipe grates/chafes against the the wooden joists where it passes through them. If the plumber that installed the system took time to ensure the pipes weren't trapped, it wouldn't happen. Unfortunately its damned difficult to put rigtht unless you fancy getting the boards up. Its a nuisance clearly but it does no harm - in spite of the alarming noise.