I think I have a motor that is going out. When I start it, after 3 seconds from when it fires, it makes a knocking noise from the bottom end. The knocking lasts about 2-4 seconds, then quits. I can run it for however long I need to, and it won't knock at all. It only does it right after it starts, especially when it's cold. It's an old motor, been sitting most of the last 8 years. Has new oil and everything from when I first started driving it again a month ago. Bearings going out maybe?