Hi and welcome to the forum!
First, I have to say that it's only the old machines that have any worth and are worth fixing. The new ones are pretty much considered disposables and not worth fixing. I would pay to have it repaired before I threw away my money on a new one. I have 7 machines, and they're all from the 40's, 50's, 60's and 70's.
If it is the timing, it may be best left to a professional, but if you think you can DIY, I'll post a link with instructions.
First, have you oiled it and put a new needle in? The needle could be bent or not be pushed up far enough. A knocking noise is never normal. Is the bobbin in correctly and have you tried rethreading the machine? If the bobbin isn't seated right, could be the problem. Even a piece of thread that got stuck in there (the housing assembly) could cause your problem.
I'm only asking because sometimes it's somthing simple and those things should be checked first.
If it's none of those things then here's the instructions (you'll need adobe reader):
http://www.tandtrepair.com/6233freefiles/HookTiming.pdf Good luck!