While driving WOT, a passenger sitting on the port side rear heard a "sound" and looked at my RPMs...noticed the port side was running approx. 1500 rpm lower than the starboard engine. I did not notice much difference at the time since it was pretty windy and I was fighting to keep it straight. After slowing down, I accelerated to half throttle and noticed my power loss. At idle, RPMs are the same, but as I accelerate, the difference is about 1500. It appears max RPM for the port engine is at 6500. With port engine running only, I can hear a faint rattle (I guess, as both engines sounded different when new). No fluid loss that I can see, oil level same on both engines. Any ideas what to look for?