Desperate housewife! Noisy central heating/boiler issue.

We have a Worcester Bosch Greenstar 24i Junior boiler, which is in the kitchen. It is 8 yrs old. It had an external expansion tank fitted, due to access problems, about 3 years ago and worked fine after that. Earlier this year it was serviced and the expansion vessel was recharged, the condensate trap was cleared, a combustion seal was replaced and a scale reducer fitted. We noticed afterwards that it was a bit noisier than before but as it was only when using hot water it wasn't a problem.
In September we had the bathroom, which is downstairs, radiator replaced with a heated towel rail and had the TRV taken off to make it the 'open' radiator and put the TRV on the hall radiator. Now we have started using the cental heating and, as the house warms up and the upstairs radiators switch off, there is a loud noise of water flowing, under pressure it sounds like, through the pipes that supply the upstairs and also supply the bathroom. The pipes are in a corner of the kitchen. The noise of water flowing through the towel rail is also loud. Added to that the constant humming/throbbing noise from the boiler and it's quite noisy in the kitchen. It's driving me absolutely crazy and also it's rather alarming!
The installer came and listened and said it's normal, it's just water flowing through the pipes and not to worry, but it was never noisy (or alarming) before. I have bled all the radiators and turned down the return feed on the towel rail slightly (probably shouldn't but I'm desperate) but it's made no difference.
I don't know if the cause is the boiler, the new towel rail, changing the 'open' radiator or something else. I do know that it's only October and I can't put up with this all winter!!
Any help/ideas/insight would be gratefully received.

Are the TRV's 'Bi Directional', i.e. can the water flow through them in either direction? Some arent, and need to be fitted on the 'Flow' pipework, as they close down onto the incoming flow of water. If fitted the wrong way wrong this can cause an almighty racket in the system, although shouldnt happen when using HW only, unless there is an additional issue with the Diverter Valve.

My Girlfriend was renting a house, WB boiler installed before she moved in and awful noise sometimes from the system, her Landlord didn't believe her, until the Neighbour rang him directly to complain about the racket! Personal opinion was TRV's were fitted the wrong wy round and as they started to try and shut down as rooms reached temp, the noise started. (Didn't help the Roomstat wasn't working correctly either, so boiler was often on when it didn't need to be and house was cooking.)

Good afternoon Tiga,

Do you have pressure in the boiler? Could be a TRV, especially the one that was moved. It could also be a balancing issue, depending on who carried out the work.

Thanks. I will check the TRV valve later but, the radiator that it's been moved to is not noisy nor is the pipework going to it. The system was not balanced after installing the towel radiator.

Sounds like. the internal auto bypass is jammed/blocked closed

Thanks picasso, I'm not familiar with that component but it's worth checking out.

Has the pump speed been left on maximum?

Hi Alec_t, I think the pump on the boiler is a fixed speed pump.