New radiator water dribble noise

I've recently installed a new radiator, a first time job for me. It's running fine, very hot, but when the heating first turns on it makes a slight dribbling/bubbling noise, not constantly, just on and off, until it's warmed up.

It is a Combi system and I've bled all radiators now and only water comes out. It's a minor issue as you can hardly hear it but its just enough to annoy me and make me wonder what is causing it? The old radiator it has replaced used to make fairly loud clunking noises when heating up, that would wake me up, so it's a big improvement on that.

TIA

 

Any ideas? It sounds a lot like this other thread:

https://www.diynot.com/diy/threads/radiator-trickling-sound-but-wont-bleed.565732/