Engine Snapper RER Knocking Sound

I have a Snapper RER Riding Mower with a 15HP Intex that has recently developed a knocking sound. I got it started before I took the video and it made the knocking sound, so out of fear of ruining the mower I quickly cut it; and I haven't been able to get it to start since. I have attached a video while cranking it. Sorry about the bad quality, I shot it with my phone. If any of you know what could be causing the sound, I would really appreciate it.



Thanks,
Ben