Hi I have a loud banging noise coming from my Baxi Duotec it seems to be coming from the Main Heat exchanger area, pressure fluctuates in time with the banging , it started last night. after the boiler was running for a couple of Hours, the pressure was low because I had bled a radiator, I reset the pressure which I thought had solved it .

but it happened again today the Boiler had been on 5 hours before it started ,when I turn the hot tap on is stops it, then it runs Fine. it seems to be be after I have turned the temp down couple of degrees then turn it up again , hot water if fine boiler is 4 years old and serviced annually, Baxi came out and repacked the diverter valve couple of months back Radiators are hot
here is a YouTube link of the Noise
https://www.youtube.com/shorts/NS7akauirlI
Boiler noise

Very odd – more like a rattling to me – sounds like something mechanical is loose and flapping about…

Not a plumber but have you checked the condensate pipe? Mine was similar when there was a build up of pressure due to a blockage where it was connected to the drainage pipe.

A solenoid firing erratically?

I have the same thing from time to time, and also the pilot light seems to fail to ignite. I had it recently serviced and told the engineer who was unconcerned, said it had something to do with air.

When it happens, usually if i switch on the heating and run the hot water, it really sounds like theres someone in there trying to get out 😆  and the first time i thought is this thing going to explode 😯

I switch it off at the mains, keep the heating off then run the hot tap. The boiler lights and the pressure shoots up, but no hot water comes out. The temp gauge goes to max(9) but then slowly comes down. I just spend a few minuted fiddling with it, tap on, tap off, see it it lights, then switch the heating back on and it seems to sort itself.

It has in the last 6 months happened about 3 times, and each time a bit of fiddling, waiting, system on system off and it rights itself.

The fact the engineer was unconcerned made me thing its maybe to do with the weather or air pressure or something, but nothing to actually worry about. Last time was about a fortnight ago, and the same fiddling about brings it back on line.

just checked the condensate pipe cant tell from outside but pulled the pipe off from the boiler end and no water came out from the. boiler,

Dyna-ti. does yours make the same noise as mine Ta

Could be kettling. Boiling water inside the heat exchanger. If speeding the pump up, or dropping the ch temps stop it then it might be this.
If it doesn’t do it for the first hour or so until the system is fully heated up it might be this.
Its ch fluid boiling inside the heat exchanger, due to calcium carbonate build up.
If it is, try Sentinel X200. Close the in and out valves on the towel rail, syphon out 2ltr of fluid witj a hose. Fill hosr with water first, then ram it down into the towel rail
Add the x200,, reseal the towel rail, pressurise the system to 1bar and fire it up
Takes 3 to 5 days to work,