89 SN trans noise

Hey y'all!
Last night I swapped the trans fluid in my 89SN. My father underneath the boat hosing the shaft & strut bearing I let the boat idle in forward gear to get some circulation and a little heat in the fluid before I sucked it out. When I knocked it in gear it almost sounded like it had some pebbles rolling around in the case. This is the first time I've swapped the fluid like this and the first time I've heard the trans in gear unloaded so this could be normal, just curious what the consensus is. My worst fear was that something is fixing to break in the near future.
The trans otherwise performs flawlessly and has the typical PCM40 1.23-1 whine or whirr that they're known for.

Thanks in advance!

throttle out wrote: . This is the first time I've swapped the fluid like this and the first time I've heard the trans in gear unloaded so this could be normal, Thanks in advance!

What you said

It's normal under those conditions

8122pbrainard wrote: Sam, get it out on the lake for a good test. The rattle you are hearing could be normal. At a low RPM, the planetary gear set will rattle. Did you happen to notice the RPM?

Agreed .   You tested with absolutely no load on the trans. The prop in the water produces a large loading factor at even 500 RPM. With boat out of the water the shaft is prone to unload and your hearing noise as Pete referred to. Feel the noise will go away when you put it in the water
Reply back after your lake test please

Hi Guys,
Thought I might ask a question here even though this trans noise flush has seemed to work itself out.....

I own a 1980 SN and had a similar noise / slip when I was running it on my main voyage a couple of weeks ago. Flushed once and replaced the fluid once. Additionally I have replaced the throttle and trans control cables and related misc parts. Upon further inspection, I noticed the trans cooler (5" long with 1" barbs and 3/8" npt fittings original equipment) was slightly damaged due to the previous owner over fighting the cooler bracket, potentially compromising the trans cooler flow..... I have elected to replace the cooler.

My question is... might a 9" long cooler be better than the 5" one, all things being the same?
I realize these things can't be generalized, but cooler, trans-wise it would seem that cooler is better.