I fixed my lawnmower that suddenly was rattling and very shaky. I was able to quickly eliminate the heavy vibrations that were making my hands go numb on my push lawnmower. This video should help troubleshoot the problem you might be experiencing as well.
If your lawnmower is shaking or has vibrations so heavy it is hard to use, than simply tightening up the blade could be the solution. A loose blade can cause your whole lawnmower to rattle and shake violently. Very easy to fix.