Just in time for the cold weather

Heating was on at 4 yday all fine. Although I thought I detected a faint knocking noise in boiler/pipework bear boiler
Yday evening downstairs rads didn't heat up.
Leg of first leg was lukewarm . Switched off heating . Bled radiator . No air came out. Switched heating back on .
All worked again
This morning all worked . Now again downstairs all off. Blew all rads. Turned off upstairs thermostat. Still nothing . Can still hear a faint occasional knock.
Not sure if relevant but both valves on nearest rad were changed about 6 weeks ago
Anything else I can check or should i just ring a plumber