Difficult to track down noise in heating

Struggling where to start with this one....

We have a Baxi Duotec Combi boiler. All radiators with the exception of the hall and towel rail have TADO valves on. The hall has a normal TRV (fully open) and the towel rail doesn't have any TRV. Heat for these two is called for by the hall thermostat (also a TADO) but obviously they will also heat up whenever any other room calls for heat.

At 6.00am in the morning the hall thermostat triggers the boiler and the hall radiator and towel rail start heating. All the other radiators are off till later in the morning.

In recent days, at about 6.10am (10 minutes after the boiler fires) there is a noise - sort of a metallic vibrating noise. It's not a particularly loud noise but loud enough to wake us up. But can I hell work out where it's actually coming from. I stand at the top of the stairs and it sounds like it's coming from downstairs - I go downstairs and can't really hear it. So I'm guessing it's somewhere in the pipes? And remember at this time there is only the hall radiator and towel rail on. The noise eventually dies down and stops but I don't know what's making it stop.

But if i try and replicate this during the day (letting everything cool down for a few hours then switching on the same two rads) the noise doesn't appear, or if it does it's not as noticeable.

The pressure is fine in the boiler and I've bled all the radiators, although the towel rail needed bleeding again a week later.

Don't know if it's a coincidence but both those rads make quite a bit of noise when they first start up - I've added a YouTube link below to hear them both - you might need to turn the volume up. Again - sounds metallic.

Not necessarily looking for answers on what this could be but would appreciate any pointers in how to try and diagnose it.

Thank you.

Go round when the noise starts and turn the valves off. Maybe a loose jumper in the valve.

Could be due to pipework expansion after low night temperatures?

Thanks for your replies guys - for some reason I didn’t get a notification that this had been replied to.

An update on this. Normally all the Tado valves and hall stat are set to Frost Protection at night which basically switches everything off. But last night I set the hall stat (controlling hall radiator and towel rail) to 15 degrees rather than switching off. No noise in the morning....until the next radiator (lounge) came on thirty minutes later then the noise came again but was still audible in the same place as before - couldn’t hear it in the lounge itself. It almost sounds as if it’s coming from a radiator that isn’t even on in a different part of the house. But appreciate it could be pipes in that area.

So as Alec says perhaps this is to do with temperatures in the pipes. But if the pipes have been warmish overnight because the hall stat was set to 15, how could there still be cold in there? Just thinking aloud!

So tonight I’ll leave the lounge at a low temperature and see if the noise coincides with whichever rad comes on next.