Cant figure out where this sound if coming from... Noise goes away when I give it throttle and its not too noticeable when the engine is warmed up. I'm heading to the boat Friday after work and some direction where to start diagnosing would be great.

408 sbc startup noise - YouTube

You sure it's not coming form the alternator. Seems to be allot louder, I can hear the valve train but the noise appears to be coming from that pulley at least form the video. My .02c

Easy stuff first. Remove those belts and restart. Next remove the starter and make sure that is hanging up and bouncing off the flywheel. Drive bearing inspection?

I agree... the belt looks really loose too... the alt is pretty old too... Its diffidently not coming from the back where the drive is... loudest point is on the front... Ill take the belts off tomorrow and go from there then.

Really hard to tell on a video. Pick up a cheap stethoscope, great tool for what you are chasing

Right on... Ill pick one up tonight.

Alt belt is flapping like a butterfly's wings. Fix that, then go from there.