I suppose its ok as long as nothing is getting hurt when its making the noise. I just turn up the stereo so I dont hear it. It would be nice to know what it is though.
That could be an issue...but in my experience it cause grinding when shifting...not the sound you describe. I had the same sound and was a little low on fluid so that is why I suggested looking at that first.
Some people may shoot me for telling you to try an additive but try some Lucus oil and see if it quiets down.
Just to give an update, I tried some of the suggestions above :
I added more dextron III up to 4 qts to try and eliminate the 5th gear noise, and it was still there in the morning. Also noticed the 1-2 grind came back and shift effort increased.
Today I drained the tranny again and went with the lucas automatic transmission 'fix' along with Dex III for a total of 3.0 litres.
It shifts easy. no grind 1-2 but was dissapointed to still have the orginal noise in 5th gear. I guess ill see it it improves over the next week, but i feel that if the lucas isnt going to help noise in 5th I might as well go back to staight ATF, perhaps give GM syncromesh a shot....