thanks, for your advice

I've tried some stuff to begin troubleshooting. it looks like it's actually not the pipes. I got rudely woken up last night at 05:00 by the noises so I got fed up and took action. First I shut off the radiator I thought may have been the problem. I did this by turning the thermostatic valve all the way to the off position - it had been fully open.

That didn't really change anything - though at first the noises weren't as loud. So I shut off the other radiator. That did the trick. But unfortunately, it's too cold to go without radiators - so I had to turn one back on, but not all the way. And guess what? The noise is back, a bit quieter, but it's back.

Looks like I may need a plumber after all.