Noise on my 2004 200HP 3.1 VmaxOX66

I am hearing an intermittent noise I was hoping someone could help me identify. Running at about 3200 to 3500 rpm.....trim motor up appropriately....definately not too high......then instead of that consistent purring, I get a different little louder almost flapping noise...in fact I thought at first it was the strap from my Bimini flapping in the wind.....it wasn't......no change in performance at all....hit the throttle and it goes away.....trim down all the way and it goes away...do nothing and it goes away...then comes back......goes away...doesnt it do it for a while.....then does it again........pressure stays the same......not a pounding...kind of a louder purring flapping noise.....wierd......any ideas? Any would be most appreciated....

Bob

Cowl rattle? You might adjust the catch's to hold the cowl tighter..

You know what Mike?.....that just could be it.....Boat is at Jet A Marina, for some other work and I mentioned it to them....I will call them tommorrow and have them check it....(great outfit by the way) Thanks for idea! Hopefully that is all it is....

Bob