What's wrong with my power shower? ?

I had a power shower installed 2 years ago with a refitted bathroom.

The shower has been fine until I started putting my central heating on in the last couple of weeks.

The shower makes a groaning sound before releasing any water but then seems fine. However when my other half then has his shower, the groaning is intermittent and the water pressure dies off.

I'm sure it's a simple thing (air lock maybe?) but any ideas welcome.

Thanks.