Well, I'm having problems watching your videos and not sure why unless you are on a very slow connection. Anyway, first thing to do is mount a fairly heavy blade in the chucks. A heavy blade so that tension doesn't break it if overdone. Also, taking all the slack out of the machine will quiet it down some. From what I could hear your saw has some noise but it could be because you don't have a blade in it or tensioned. If after doing that you are hearing any noise, it's probably coming from the tension rod hitting something because it's too loose or the upper link arm is contacting the underside of that cover. Here is what you need to do to fix that. It's a common problem with these saws.
Here are pics of what needs to be done. On my saw, when I had it, I had to actually grind away enough of that cover that some of the groove was actually gone. What is happening is the link coming from the rear of the saw to the front upper blade mechanism is hitting the underside of that cover at high speeds. Also, if you have too much slop in the tension rod it will flail around above the cover and hit the cover. It's just a matter of relieving the cover's underside and fine tuning the tension rod on the upper side. Once you have managed to do all that, you shouldn't have any more knocking.