My new mower was working fine this morning but an hour later I was mowing my neighbours lawn, because his machine broke and I heard a strange noise, I was cutting tall grass and thought I should get my side sidecharge, and that's when I heaard the noise. I turned it over and the bolt was loose, so I tightened it and started it again, same thing so I undid the bolt and checked it, everything looks fine, so I tighten it again, and it loosened right after I started it. What is going on?

You can either try to torque it to the factory specs and see how it does or use a little loktite on it so it doesn't vibrate loose.