I know you're not gonna take any advice from a DIYer (rightly!) BUT my system is very big. When we first moved in and I needed to drain down and fill up I couldn't understand why the pump was making a grinding/buzzing sound and nothing was getting hot.
Was in the middle of winter and I knew less then than I do now (still not much more but I do know about MY system) so I called in a local plumber. turned out I'd got a good'n as for whatever reason he didn't charge much. But while obviously he knew straight away it was an airlocking issue to fix it required:
- running the pump for a few seconds at one speed then changing the speed then turning it off and waiting for a bit
- opening and closing the MVs with few seconds delay
- getting me to open up and close rad vents and valves
- generally seems to just keep "messing" with the flow
- bleeding pump and anything else that could be bled
- cracking open a compression end stop at the top of an upright pipe in airing cupboard
eventually we started to here the noise of bubbling rushing up the pipes and a 20 minutes later of me fretting eventually got the airlock(s) out.
I guess every system is different and mine now has a lot more bleed off points so I don't need to do the above too often.