Glow-Worm Fuelsaver Boiler 45F and Central Heating?

Hi

Really hoping someone can help me.

I have a Glow-Worm Fuelsaver Boiler 45F which is approx 18 years old.

Over the last few weeks I have been getting a rumbling noise but it's difficult to tell if its coming from the boiler or the pipes above.

The noise occurs when the boiler has been on for a few minutes and is just about to cut out as it reaches it's max temp. The rumbling starts quietly each time and then builds to a very loud noise before the boiler cuts out, only to repeat this a few mins later when the boiler comes back on. Not sure if boiler is cutting out early because of this as water and heating are still nice and hot.

It was first bad when it was just the hot water circuit working but now it also does it with the heating but not always, always does it when hot water only is going.

I called a gas repair guy who said it was sludge in the system so have had the whole system drained, de-sludged with Fernox F3 Cleaner, flushed through and refilled with Fernox F2 Silencer and F1 Protector but it hasn't made any difference to the noise.

Gas guy is wondering if it's heat exchanger or a new boiler needed. Dont' know if I can get heat exchanger for the boiler as old and don't want to go to expense of new boiler with out knowing exactly what problem is as thinking both these things could still result in no change in noise.

Any ideas please?

Thanks