I have an 81 C-30 with the 5411. When motoring it occassionally makes a "thrumming noise" and vibrates somewhere in the drive train. If I slack off on the throttle and power up again it will smooth out. It starts again on seemingly a random basis, sometimes in a few minutes, sometimes not for a half hour or so. I'm wondering if it could be my cutlass bearing, my stuffing box, my shaft alignment or maybe a slipping transmission. I first noticed it when we were in SF bay in some confused seas. I chalked it up to cavitation caused by the seas but have now experienced it in smooth water as well. Any ideas on likely causes or how to diagnose it?