There is a rubber on the stop - right? A "thud" should be noticeable, after all, the OTT spring has just kicked in !!

Keep in mind the purpose of the OTT spring. It is there as an "assist" to depress the clutch BUT it (the spring) can't be engaged at all times or it would hold the pedal down enough to hold the TO bearing in contact with the PP. So when the pedal gets back up about 1" from the top the spring crosses a threshold whereby it now pulls the pedal UP - with quite a bit of force (if you've ever tried to stretch one of those buggers you know what I'm talking about). So yeah, there's going to be a "thud".