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

Gavin
January 2012