How can i stop my Karcher 390M pressure washer Pulsing?

Hello.

I think I'm probably asking a similar question to others but I too have a pulsing karcher when i release the trigger and no water is flowing. It started at 10 second intervals but is now pulsing every 2 seconds. I've stripped most of it down but can find no leaks. I have observed a piston like switch moving whilst the unit is pulsing and the detergent pick-up tube is not sucking.
Any advice would be grateful. Also I'm struggling to find this model no. on many 'spares' websites. Is it similar to any other model?

Thanks in advance.