Right, let's start at the beginning.
Before it started knocking did you regularly check the oil level?
If so, what grade/type of engine oil did you put in it from new?
If not, how much oil did it need when you "topped" it up?
If the oil level was on it's lowest mark before you topped it up and you have been running it like that for some time, then I would suggest the low oil level has accelerated engine wear- causing the knocking you are hearing.
Do you use the mower on a slope for extended periods? If so, insufficient oiling of the engine whilst cutting at an angle has caused the same problem as above.
Finally- are you sure it's the engine that's making the knocking and not the blade fouling the inside of the deck?
pleasant
October 2011
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