The mechanic who worked on repairing our broken drain plug said that in his opinion the drive unit is shifting "firmly" and almost banging into gear (fwd and reverse). There has always been a slight "clunk" but now he has me wondering if something big is wrong. A side from a gear oil change the drive has never had any service performed (since we have owned the boat the last 5 years). Before we have the unit removed and inspected (i'm sure it won't be inexpensive) is there something that we can check ourselves? The drive unit and engine are 2013's. Appreciate your thoughts.