Thanks man!
It could be the cheap recoil mechanism, but it doesn't look like that's the problem. Maybe I can put some WD-40 on some points there (on the white plastic bits under the hood) to see what happens? I'm definitely going to check on rod knock, but I'm still waiting for the delivery of...
First, this lawn mower did not start at all. I replaced the spark plug, cleaned the carburetor, changed oil and gasoline, cleaned the flywheel (there was grass stuck to it, removed it a bit with a screwdriver) and the engine worked (hooray!).
BUT: the engine now makes a rattling noise. It is...