Potterton 40L fan issues

Was recently woken up by my boiler making clunking screeching noises. After speaking to local heating guy, he thought it was the fan.

We have now changed the fan, only to find after the boiler kicked in for a few minutes the fan started to make the same clunking screeching noises!

Anyone have any idea what the issue could be?

 

Who changed the fan?
Was the fan gasket replaced?

 

I fitted myself including a new gasket. Confused why a new fan is making the same screeching noise as before, when it’s a new fan. Makes me thing could be something else?

 

The forum rules dictate that no advise is given regarding combustion chamber related issues...certainly not diyers replacing fans.