Back boiler heating system - Noise when pump is off

I have an open fire with a back boiler.

Lit the first fire of the year yesterday and once the fire got really hot, at the kind of temperature that the pump would usually kick in, there was a gushing noise from the pipes, and loud rattling, as though somebody was shaking them rigorously. The pump was however turned off.

I went to the hot press and the noise was pretty loud up there too.

The water in the tank however was not hot.
It would usually be boiling with the fire going.

Any ideas before I call a plumber?

Check to see if the spindle in the pump is not stuck. This can happen when the pump has beem idle for a prolonged spell. This spindle is attached to the motor that in turn operates the impeller that moves the water in the pump. The spindle which may need to manually turned is accessable through the blead screw on the front face of the pump.
The gushing/rattling noise is water boiling in the boiler and this boiled water circulating around the piping circuit to the hot water cylinder( gravity circuit, hopefully you have one!!)
You should also check to see if the pipe thermostat operating the pump is working properly.
If all this fails call a qualified plumber familiar with solid fuel open vented heating systems. Hope this helps