Baxi screeching noise

[Mojos]

BAXI SOLO 3 PF 70 (4yrs old)
Central Heating works, Water heating works. Set to switch on automatically 2x daily.

When it starts up, a terrible screeching sound can be heard in the baxi unit. It sounds as if it could use a good squirting of WD40, but I believe it might be the pump(?) not getting enough water through it.
Within half an hour it settles down and seems to behave itself.

I recently put in new taps and shower hose in the upstairs bath which required me to turn off both hot and cold supplies.

Thinking it might be air in the system I tried the advice from this message:
//www.diynot.com/forums/viewtopic.php?t=9660
Now the screeching is intermittent for half hour and then settles down. But its still noisy and probably wakes up the neighbours as well.

What do I do next? Baxi specialists want 200plus pounds just to ring our door bell, so that's not an option for us. Thanks in advance for any advice.

 

[ChrisR]

Pumps make ghostly groaning noises, the banshees are usually the fan's fault. There's a small extra fan on the same shaft as the main one, which is there to cool the motor. Sometimes it comes a bit loose and scrapes the metalwork.
So you need a corgi, but a fairly friendly one. Many would just say you need a new fan, which is £££. I've replaced grub screws and the like in several.
Can't think of anything else which might screech.