We recently had our boiler replaced with a heat pump. To help the circulation of the water in the radiators, the engineer installed an external pump.
Most radiators now make a constant running water noise, probably due to the circulating pump being set too high?
One of the radiators has now started to make a new noise: it sounds like filling up a bottle. There is no other way I can explain it. It lasts a couple of seconds (until it sounds "full"). After a while it will start again. It goes on for a few cycles. Also, possibly related, the radiator is now only heating on a vertical band near the IN valve and at the bottom. A bit like this:
Before the installation of the HP, the engineer cleaned the whole network with sludge remover. The affected radiator was working fine at the end of the last season (apart from the common running water noise).
I tried to bleed all the radiators in the house and only one had air in.
What can be the cause of the weird (filling up) noise?
UPDATE:this morning I noticed another radiator making the weird noise. So maybe is something in the system?