that sound is from the electric-magnet at the drainage-valve.
When the brewing stops, the fluidsystem would rinse through the nozzle
in the front and it would give many drops of coffee.
So this problem is solved by the drainage-valve, which lets the water out and air in
to stop the tripping.
The electric magnet knocks a steel-needle at a plastic cap which is mounted to
a metal axle in the valve which then opens.
Sometimes these Juras make awful humming noises while the electric-magnet
has current, too. - when these Juras get older.
As long as the knock sound and the humming is only hearable while the drainage-valve has
opened evverything is okay. :)
Friendly greets and good luck. :)