1999 tranny noise

Need help! When gears shift there is a revving noise sounds like 2nd and 3rd. Car drives normal most times. Shifts normal only it maybe slipped twice. Had tranny flushed but seems its worse now when you go. Sometimes no noise and drives fine. Took it to a tranny shop and they scanned it no codes. Friends say its a grinding noise but not sure. Rpms are normal. Car drives good just the weird noise. I read something about ring and pinion gear. Don't know anything about it. Does anyone think that's my problem? Would i need to get tranny rebuilt? Plzz help!

Oh yea sorry it's a 3.0 v6 dohc 24 valve engine with only 70000 miles on it.