Heating issues. Had new water pump fitted 3 weeks ago, new 3 port valve 18 months ago, boiler is 4 years old. I balanced radiators when new pump was installed and for 2 weeks or so the heating and hot water worked a treat. 3 days ago, woke up to find neither working properly. Timer comes on, boiler fires up but radiators downstairs not getting warm. Hot water not great. Balanced radiators again and all was fine but last two days every time I turn heating on nothing warm downstairs in spite of balanced radiators two days ago working fine. Pump does seem to work fine when it runs quietly but each time it starts there is a whirring noise and you know it won't work properly. Any ideas?