With Brownster's help I tracked it down to be coming from the internal rear derailleur cable rattling around inside the frame (probably not an issue with the electronic shifting versions). Armed with google and my awesome backyard engineering skills


Basically 2 zip ties, one at each end of the RD cable to keep it a little in tension. A little bit of rubber under the zip ties to stop them slipping along the cable. I was tempted to pull the BB apart to see if there was a better way to do things but the bike still feels too new so I've stuck with the non-intrusive solution for now.
The bike is pretty much silent now
