Twin 454 420's and when I'm cruising around at 3k everything sounds fine. When I increase throttle I hear a whistling sound what seems to come from the starboard engine. If I accelerate more it gets louder, all gauges read normal and boat seems to run fine. It did it last year then went away and this year it did it after running in a poker run. Ran fine in poker run then on way home heard it. So it comes and goes.

Any ideas?

I would have to go with Vacuum. Check to see if you have vacuum secondaires on the carbs, if so it would most likely need new boost diaphragms. It could be just be a base gasket or intake. Start with spraying WD-40 while it's running. If the idle changes that is the point. If you do not find anything that way, then look at the carb secondaries.

I never noticed the sound when I was running my stock 420's (800cfm mech sec holleys). My whole setup is way different now with roller cams, new intakes and carbs (quick fuel 800's). I now have that same hissing/whistling sound. My friend with a 502 and 830 has the sound also. Always wondered what caused it...