Potterton Prima B - pump noise during timer off mode?

Our Potterton Prima B is controlled by a Randall 4033 Programmer which must be at least 15 years old. The boiler is about 12 years old.
We always run the boiler in Timer mode. We have suddenly noticed that there is a pump noise when the timer is in the Off part of its cycle.
We have tried switching bewteen Timed and On mode and this seems to make the noise go away.
Does this indicate a problem with the controller? If so, what is the cheapest way to fix it? My wife and I have both been made redundant recently so cannot afford expensive repairs.