Vibration/Noise

[jamanati]

Hey all, well, like the beginning of any boat season, new problems seem to arise. I'm no different, and any help or suggestions would be appreciated. Went out for the first time this season just the other day. Ran fine for the first half hour, stopped into a marina for a while, and later that day, on the way back home and after about 20 minutes or so, a strange vibration/noise started to occur in the rear of the boat, getting more noticeable and louder as I drove. I was pretty close to home port and had guests onboard so it wasn't the time to keep driving the boat to diagnose it further. But this is what I have so far.

Boat: 95 3200 Scr, twin 5.7 mercs with bravo 2s.
Winter service included new gear oil in both drives, props cleaned and balanced, engines services, etc. The usual routine.
Engines tuned up, tach'd at correct rpm. All fine there.
I did put in a new merc gen2 shifter, with all new control cables, so thats new this year. (wow, big difference in shifting by the way).
Engines tuned up, new caps, rotors, plugs, wires, and all new drive belts.

The vibration is *really* noticeable when I was up on plane, I was going at about 3200 rpm at that time When I dropped down 2000rpm, the vibration was still there, but at idle in the no-wake zone, the vibration was very faint, almost unnoticeable. While idling up the river, I put each engine into neutral and rev'd up each engine to 3000+ rpm, no vibration noticed.

In the slip, with engines off, tilted the drives all the way up into trailer mode, checked props for any damage, they looked fine and clean. Dropped them back down, started each engine, placed one drive at a time into gear (while in idle and still in the slip) and tilted up the drives a bit, past the point where I had to press them into trailer mode, but not that far up, to see if any u-joint issue, again no vibration noticed. While in the slip, couldn't produce any situation to make the vibration heard.

I am thinking it may be something with the prop (out of balance, bent a bit, something like that), or something internal in the outdrive. Other than than, I'm asking here if anyone may have any other ideas or suggestions for me to look into. Thanks.