Cheers for the suggestions, give knob etc is fine, but I think I may have guessed what it is, the slide itself doesn't seem as solidly fixed as it should be, there is just a slight bit of play on it. When the machine is running and I very gently press on the front of the slide the machine is silent. The slide on the Santa Maria is solid, no play at all. So I think that's what it is, it just seems like something needs tightening.