Knocking sound after clutch and electrics work

[widung]

I just had an experience that I'd like to share with members if that can help someone:
I needed to change voltage stabilizer, so instrument panel was loosened for access as stabilizer was mounted on rear of oil gauge. Clutch slave cylinder changed at the same time.
When everything was assembled and with engine running there was a knocking sound from the instrument panel/gearbox region.
I crawled under the car, expecting something in the slave cylinder being too close to the body or gearbox - no.
I finally managed to localise it as coming from the vacuum tube inside the engine room and believed first that the non-return valve was faulty. Thinking further I remembered that I disconnected the 2 vacuum tubes to the Headlight button on the instrument panel. I switched the 2 rubber tubes and that was it! No faulty non-return valve!
Hope this helps someone!
Jan