Check glow plug. How bad is the knocking? Is the knocking more sounds more like valve lifters? If so, you will have to check valve lifters and make sure non of them are defective. It is called roller lifters. Another cause of knocking is a failing oil pump or when the oil pressure is down the recommended factory specs.
Bad fuel can also make diesel to smoke, but normally smoke is a sign of failing glow plug or a motor loosing its compression. Remember diesel engines are high compression engines. So, low compression can cause unburnt fuel.
Good luck to you..
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