The pressure is fine (15 psi) and it heats the house fine. I would think the water needs topping up except for the fact the pressure is optimal. Any thoughts on what I can check/do? The sound is startling when it first kicks in and wakes me up in the night. I should also mention that this is a more recent sound. The previous 2 winters, it would start with more of a 'bang' sound to the pipes. Since turning on the system this winter, it's no longer a 'bang', just more like 'whooshing' water. When turning it on this year, we also discovered that it was not properly wired into the electrical breaker (problem resolved by a certified electrician).
The bang is probably water hammer - there are various ways to deal with that.
The whooshing sound, may be caused by bubbles of air in the water-pipes. The normal way to deal with that is to bleed air out of the system when it is not running. The system will have bleed valves. Their location and type depend on the type of system and vary by locale/country. You can obtain automatic air vent/bleed devices that can be plumbed in to automatically bleed air out of a system (the drawback might be that you are not alerted to a problem with air getting into your system)