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Have the lucas or the oil change made any type of difference in the sounds of the knocking?
What grade fuel are you using?
If the lucas did make a difference that you have a clogged/dirty lifter(s). Lifters shoot up oil and keep the motor lubricated, if you have a clogged or dirty lifter then you will not get enough lubrication. This will cause a knocking noise from the engine. Often times that can be fixed using a engine flush or engine treatment. However if it does not work, then you may need to replace the lifter(s).