Have you changed the needle since you purchased it? A needle should be replaced every 4-6 hours of stitching so for each garment sewn as a rule of thumb. So try replacing the needle, cleaning and oiling the maachine as per the manual. Check needle is in the correct way, it is easy to put in backwards if you aren't familar with them, flat shank to the back and there is usually a scarf (cut away area) at the back of the needle which is where the rotary hook under the needle plate comes around to catch the upper thread during the stitch process. So if needle is in backwards this will cause stitching to fail.
So elimate the needle firstly.
If this fails and the knocking noise continues, then it could be that the timing has been knocked out. It can happen if you sew over a pin and hit it hard (or the needle strikes the foot if stitch setup is wrong for the foot being used). In this case, you'll need to take it back to the shop for the serviceman to fix.
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