I have a used la Pavoni. lately, when I brew espresso, I hear a sound like water splashes or bursts while pulling the shot--I really do not know how to describe it in a better way. When this happens, I get a bad shot. This started to happen often now, though not always. This may happen once, and the next shot is made by the same coffee and grind, and it turns well.
Any explanation? Does this mean there is a problem in the machine that I need to take care of?