It would help if we knew what engine we are diagnosing
I would put my money on a leaking head gasket between the push rod tunnel & the cylinder
The quick & dirty check is to run the engine for 10 minutes or more, turn it off, count to 10 then pull the dip stick
If smoke wafts out of the tube then the head gasket is gone
Another confirmation is checking the breather
Blowing on the up stroke indicates a blown head gasket .
A knock or rattle when the engine is suddenly loaded ( blades turned on or cutting very thick heavy grass ) or turned off .