Redring Expressions 500 SHower: Thumping sound?

Hi,

I have a Redring Expressions 500 Shower unit. Just over 2 years old. In the last few weeks, when it is turned on, it makes a thumping sound, and the water comes out in approx 1 second bursts. After 5 to 35 seconds the thumping stops and the shower works as normal.

The shower is always set to the High Heating setting. I have set it to Medium and the issue occurs, when set to low it does not.

Would appreciate any help or advise you can give me on this