Loud humming noise from hot water pump

We recently had two new bedrooms put in the loft, each with a radiator.

In the past two days, a loud humming noise has started from the heating pump in the airing cupboard. During boiler-on (hot water and/or CH) the pump is silent, it only emits the loud humming noise immediately after the boiler turns off, and lasts approx 1 minute.

We have a Megaflo unvented indirect cylinder, with a Grundfos UPS pump. The system is nine years old, installed new with the house.

I have checked all the radiators, and none need bleeding. The system pressure was a little low (1 bar), so I've let in some water to get it back to the recommended (1.5 bar).

I have also tried unscrewing slightly the large front screw of the pump to vent any trapped air. However, after only a quarter turn anti-clockwise drops of water appeared but no air was released, so the screw was returned to its original position.

Any suggestions anyone?

Thanks,
Mark

Possibly a bypass problem, if you have one.

kidgreen61 said:

Possibly a bypass problem, if you have one.

Click to expand...

I'm not fully sure if there's an existing bypass, but the two new radiators were put on the existing bedroom circuit, so to my novice eye nothing complex was done.

There is however something labelled as a balancing valve next to the pump