Unexplained noises?

Good day,
I have been experiencing a problem with my domestic heating system. There are occasions when the system makes noises that seem to indicate that water is circulating. These occasions however occur most often late at night when the system is not due to be working. The noises occur despite the timer being at a time when it is at "off" for both hot water and heating. The gas boiler is not alight at these times.
The noises come from my airing cupboard tha6t houses a Megaflow style water tank, a lot of pipework and what I believe is the circulation pump (manufactured by Grundfos). its difficult to tell but the unexplained noises seem to be coming mainly from the Grundfos unit. During these occasions, the unit is very warm / hot to the touch.
I have consulted plumbers and heating engineers about this problem but so far no one has been able to diagnose what is happening.
I would be very grateful for any advice.