My first thought is the terrible knock I had on my 1200. It turned out to be the Idler Arm bushing


If the strut nut is loose, the strut insert can rattle up & down in the tube making terrible noises. Tightening it is a very low cost fix


I don't believe changing the strut oil will make a knock go away. If a strut is bad it's like a bad shock and the body will bang against the suspension when going over bumps, even at low speed. That's not caused by the type of oil, but by worn valving. You can fit thinner or thicker oil to fine tune the damping but it won't fix worn/swollen rubber valving parts