Hi, I just had the same problem and fixed it by tightening the piston which had loosened itself from the piston rod.
This is actually a very quick and easy fix.
You need to remove the diffuser screen to access the piston, then tighten piston, which is threaded onto the piston rod. I simply tightened it by hand but for a more permanent fix, you probably want to take the piston and rod right out so you can get it tighter, or maybe use some locktite or similar.
The good thing is you can give the machine a clean behind the ears while you're in there and one or more of the rubber gaskets may be due for replacing anyway.
For details on how to get at the piston, read through this web page
http://www.pavoniexpress.com/sealrepl.html -
REPLACING THE GROUP PISTON GASKETS AND GROUP TO BOILER GASKET