We had a 2003 Catalina 350 survey and sea trial done today and there was a severe vibration and noise in the floor of the cockpit when under diesel power at about about 5 knots (full RPMs). Does anyone know what might cause this? Slower speeds the problem was minimized. The sails were stored for the season so we don't know if the problem was caused by the diesel engine or by the speed of the boat. We listened carefully in the lazarette compartments, under the aft bunk and in the engine compartment. The vibration was most evident on the diesel fuel tank under the aft bunk. It was not as noticable in the engine compartment or near the shaft. We could not identify the source. When tied at the dock we tried the engine at full RPMs in reverse, forward and neutral without the noise or vibration. Any ideas? Thanks for any help you can give us.