We have a 36 Outlaw with twin 496 HO's. When we get to about 4000 rpms, I hear a popping sound coming from one of the motors. The rpms didn't drop a noticable amount, just he popping sound. I couldn't tell which one was actually making the noise while driving the boat. The motors have about 140 hrs on them and run great otherwise. Any thoughts on what is causing this issue?