How does it sound...
 

How does it sound when valves are adjusted to loose?

Sometimes (at low speeds, with the pedal just slightly pressed in)I hear a noice coming from the back. It sounds like lightly shaking a glass cup with some nails on the bottom...

Best analogy I can think of is an old sewing machine 'ticking' a bit too loud ... like it needs cleaning and lubing. But that memory is of sewing machines more than 40 years old, since most these days use plastic gears -- the BANE of a good sewing machine mechanic!