Blazer Knocks hard when starting and on shut down

a few days ago my blazer started knocking almost like a bearing knock when i start it up and shut it down... its not there at all when driving and i have noticed no loss of power or mileage... i was hoping it was not a bearing as i just put this motor in in march... thenon saturday i raced my buddies 360 Ram... which i beat BARELY!!! and noticed the knocking stopped it stopped for a day or so and is now back again on start up and shut down... oil pressure is good and i flush the system every oil change so the screen should be clean... also thought of pre-detenation... any ideas???

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could be a lot of things. what year, what engine and how many miles? SES light on? if not, any codes pending?

94 4.3 W - no SES or pending codes

sounds like piston/pin knock.