Well after the prop mixup, we went back out to the lake on Friday, and the boat performed well. Good hole, shot, RPM's right where they should be. Ran for about 20 minutes at about 3000 RPM, with no problems. When I returned to idle I heard a rumbling noise from the engine compartment. I put the boat in forward and reverse, still same rumbling noise. I just had the impeller replaced and lower unit serviced. Temp is fine, oil pressure fine, no alarms. Any ideas would be helpful. Cheaper one's are always welcome!