okay until our oil ran out

[broadwood]

Central heating ran quietly for two years until we had two new radiators added onto the system, then we experienced very loud knocking throughout last winter while the boiler was on.
However the boiler was off from March 2018 leaving us with 500 litres in our oil tank until we turned it on at the start of November 2018 and we had no noise !
Silent unless we turned on the first rad nearest the boiler downstairs, so we didn't.
Unfortunately we ran out of oil last week, and then had to bleed the new oil supply through to the boiler to start it again and the loud mad banging is back. Our house is going on the market in 3 weeks so we're a little anxious
about what has restarted this problem. Any theories, can air enter the radiator system through bleeding the oil to the boiler, boiler is starting/running fine now it has oil.

 

[oilboffin]

If your selling then you need by law to leave a fully working heating system and you can be liable
if it's not.It's in your interest to get in an engineer.Bob

 

[broadwood]

We know this, hence our panic. Our worry is we had two heating companies in before when it was noisy, they both checked the rads were bled and replaced various parts in the boiler - just in case. This made no difference at the time. We were advised to take up all the flooring upstairs and wrap the pipes.
This isn't very logical since it can run and does silently. See above.

I'm asking for information re the change in behaviour of our system due to running out of oil so I can point the next engineer in the right direction ? but really to know they're on the right track. Before it seems we've really had a lot of general guesswork.

Googling the problem, unsuccessfully, I haven't seen our system bled from the boiler out as described on-line.
But this still doesn't explain to us why it ran silent and is now extremely loud.

 

[endecotp]

can air enter the radiator system through bleeding the oil to the boiler

No.

 

[OvertonPlumber]

Sounds like water starvation. Sticking pump.