Unvented cylinder loud humming noise

[GarethF1981]

So we switched from a vented HW tank to an unvented one about 4 months ago (rads were already on an unvented loop) for pressure and volume reasons. Ever since it's been installed we've had a loud humming noise/vibration in the airing cupboard whenever the hot water is on. We've had the installer back out who has drained down the system and done whatever he's done and it's would be fine for a week or two then back to the vibration.

The vibrations occur on the HW feed in/out around the tank but don't impact the CW feed into the tank or that pipework.

I thought it might be a trapped air bubble and I've got air vents both at both input and output from the tank so I've checked these and while there was a little air the noise is still going after they've been bled.

Looking on the forum today I saw mention that it might be a tap so I've just checked and if I run the bath tap in either the main bath or en suite then the noise goes away. This does not happen with any other hot tap. As soon as I turn the tap on the noise comes back which, at least, is progress in the investigation. Both of these taps are the standard basin mixer style tap. There are no other issues with the system i.e. no additional noise when running taps or showers or any leaks.

Any thoughts on next steps?

 

[GarethF1981]

Update:
After going down the rabbit hole I've done a few more tests and have made more discoveries.

The system is set up with a single flow/return to the boiler. At the tank there are two valves: one for CH and another for HW. There is also a 15mm return with no valve that goes to the two bath rads so they run whenever heat is called for regardless of whether it's CH or HW (a feature I think).

We had the HW valve replaced when the issue first started but not the CH one so I wondered if that could be the cause. I switched that to manual open and the noise stopped. The noise then came back when I flicked the valve back onto automatic (so it closed as there's no CH called for). Easy, said I, I'll just switch it to manual and close all the rads down until I get a plumber to come and swap out the valve. Apparently not. I closed all the rads down and the noise returns. It's only when I've got a certain number of rads at a certain setting that the noise goes away. At this point I looked at the pipework and saw that there is a reducer before the HW valve. The main flow is 28mm as is the CH but HW gets reduced to 22mm before the valve.

Full transparency - we've had an extension done with UFH which is tied into the main flow/return on the boiler. The boiler has not been changed and upstairs the tank pipework has not been changed before or after the valve except for the valve replacement and new pipe to connect to the new tank.

Is this a flow rate issue and if so can I reduce the flow rate in the boiler? For now I've got a workaround but it's not ideal to have rads on during the summer. Also, why would turning on one of the bath hot taps stop the vibration? Especially when other hot taps don't affect it. And why would this be an issue now when we had no such issues before? As I said, the pipework hasn't changed except for an extra T off the flow to the UFH which is valved and currently off.

Cheers.