Small Engines (Lawn Mowers, etc.): Honda GVC-160 lawnmower knocking sound, oil drain plug, short grass

Question
I have owned this fantastic mower for about 18 months.  Engine still runs strong but I hear a knocking sound from the engine.  Started 6 months ago, getting slowly worse every time I mow the lawn.  Already changed the oil and it was very clean the first time I changed it.  No pieces of metal either, but it is hard to say what is at the bottom of the sump.  Last time I checked it, still clean.  I have about 30 hours of operation on the engine.  Just cutting short grass.

Answer
Are you sure the noise is not coming from under the deck.  A loose blade, cracked blade adapter?  What oil brand and weight are you using?  Does the engine burn any oil?  Is the spark plug tip black or a light brown color?

Do you remove the oil drain plug to drain the oil?

Let me know.
Eric