=== Cause of the problem ===
My guess would be that a lot of dust and other debris has gotten stuck around something, which stops the part from spinning smoothly. It may also be that the debris has forced a part away from where it’s supposed to be.
I have another brand of a robot vacuum that I'm taking apart and cleaning now and it really needed cleaning, it didn’t have the same problem as yours but I found a few places where it looked like it would cause a similar problem anytime.
=== How to turn it upside down and it still runs ===
From my understanding, you should be able to turn it upside down by holding the wheels down, I think that’s the mechanism detecting if it’s on the floor or not.
=== How to solve it ===
If you feel comfortable with taking it apart and clean it from dust etc., that’s what I would begin with.
If you find what part it is that sounds bad and don’t want to clean the other moving parts when you're on it, you can go for just that one.
I found two guides on how to get to the [guide|88842|Cleaning Head] and [guide|88865|Side Wheels], and [https://www.youtube.com/watch?v=Uv0oQWvdpGk|this video|new_window=true] should also help you.
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If you found that a part looks weird/broken in the process of cleaning it, add a picture of it in your question and I or someone else can help you.
Hope this helped, feel free to ask if you wonder anything else and good luck. :D