Tranny Noise?

I have followed here for awhile but can't post due to work restrictions, yeah i take a break from work and jump on Wake World. This weekend i noticed a chirping noise coming from what sounds like the transmission, like something might been thrown against the outer casing? It only happened on long cruises at 30 ish MPH. If I stopped, the noise would go away for awhile then return. Never happened wakeboarding or surfing. The boat ran fine, transmission fluid is full and it did not appear to be slipping. Anybody dealt with a similar issue or have ideas as to what to check? The boat is a 2004 XStar with the 350 MCX engine.