4.3l Carb Strange exhaust noise at aprx 2800 rpms

[tahoejag]

4.3l carbed a1 gen2. While underway motor runs smooth yet at 2800 rpms I can hear a strange exhaust noise (almost like a bad gimbal growl). This noise comes and goes (surges) and depending on the water conditions/size of waves but stays constant on very smooth water. Only is noticable at this rpm. If i throtle up or down this noise vanishes. I am leaning toward an exhaust problem. I do have an issue with port side riser hotter than starboard but I have read that is normal but am also getting noise from shutters which was not noticeable before. Any ideas where to look first? I did have the gimbal and ujoints checked out beginning of season and was said ok. Basically stumped at this point what would be making this noise.