Offshoreonly.com - Pre Alpha Mercruiser help

I have a 1982 Sanger Mini Cruiser with a 260 mercruiser and pre alpha outdrive. On my last trip out I started getting a loud popping noise (sounds like a hammer hitting the outdrive) from the rear of the boat. The noise occurs in gear and out of gear. While out of gear I had my hand on the out drive and you can feel the popping noise in the drive unit. Everything works fine (goes in and out of gear smooth and pulls skiers/wakeboarders). I have had the boat since 2012 and have replaced the universal joints gimbal bearing and bellows and oil every year. Does anyone have any advise on where to start trouble shooting this noise? I was hoping it could just be a shifter cable needing adjustment. I am pretty mechanical however ever green with out drives. Thank you