1984 Honda 125m Knocks in third gear

[dmo82]

I got my 125m three wheeler running that I rode as a kid. Hasn't been ran in probably 15 years. When I was a kid I rolled it, Ever since then it started to knock when I put it in third gear. Any ideas what it could be? Where I would start with problem solving. I have a video I can upload of the sound if that would help.

[dmo82]

https://youtube.com/shorts/1bZsffezz5A?feature=share

here is the video of it knocking in third gear and then i shift to 4th and it stops