Noise from engine compartment

I have a 2000 Ski Nautique with the Chevy engine and Perfect Pass. I'm getting an erratic “screeching” noise from the engine compartment/drive train. Last time it was so loud that the guys in the boat next to the course asked me what the noise was. It only happens after a few sets and lasts maybe a second, once for about 3 seconds. It doesn't happen at any particular time, i.e. accelerating, turning, etc. There is no noticeable change in performance/vibration when it happens. I’ve gone through everything I can think of; changed the trans fluid which seemed fine before hand, changed the impeller (never ran hot), checked shaft alignment, engine oil level is OK, belt’s tight. I have less than 150 hours on the boat. The transmission does seem to get hot but I’m not sure what normal is. I checked the trans cooler and there is no obstruction. Problem I have is if I take it in to the dealer they most likely won’t be able to reproduce the noise. Has anyone experienced something similar? I’m at a loss and don’t want to end up with serious damage (if it hasn’t happened already). Thanks.

Rick,
I hadn't thought of the water pump. I haven't had the time to check it out yet but it does make sense because the pitch of the noise seems low and the reduction in RPM due to the pulley could account for this. Hope this is the case since it's an easy do-it-yourself job as opposed to working on the trans. I rebuilt my old borg-warner but that was when I had the time (before I had a wife and 4 kids).