On my 405 TD I can hear it spinning up and down with the radio off. If
its spinning its probably working. Mine comes in about 2000 rpm and
there is a noticeable performance change once it's kicked in - if you
dont see and step in performance then that's a good indication
somethings wrong. Also try and v max the car (hold onto your license
firmly) if it can't do easily over 100mph then you're down on power a
lot - could be turbo.

Hope this helps IanR