transmission fluid change, now new sound?!?

I recently changed the transmission fluid in my 2000 xstar (preditor engine, combined vdrive/trany).

Now, when I put the boat in neutral after reving it, I hear a faint grinding noise, kinda could sound like the clutch plates rubbing. Usually lasts for 1-2 seconds while the engine revs down.

I used one a tempo pump/container to siphon the fluid out of the dipstick hole. I filled until the dipstick read full, but that was about 1/2 quart less fluid than I siphoned out. I tried adding another 1/2 quart so I would have replaced the same amount of fluid as I took out, and it still does it.

The fluid I removed was dark red, no milkey or white color.

any ideas what this sound is? I already have the typical mastercraft trany rattle, and this is not that sound.