I have a new 2007 boat w/ new 2007 150HP Optimax. Only used it once last year before putting away for the year. I have noticed that when it is sitting in my garage and I spin the propeller forward it is pretty quite. HOwever, if I spin it backwards (reverse) I get a knocking sound that is consistent. THat is....if I put my finger at noon and keep it on the propeller while I turn it I get the knock at the same point consistently. Is this normal? Should I be concerned?