2001 Chevy Lumina transmission Question (planetary gear set)

My 2001 Lumina is making a whining noise and goes to higher than normal RPM when reaching a certain speed. It makes this noise in overdrive and in lower gears, but not when in drive. A mechanic told me it was my planetary gear set and he reccomended me to just rebuild transmission, becaus car has 125000 miles on it. But I do not know if he is just after more money or not. My next question is would it hurt to drive in drive and not overdrive until I get it fixed?

It’ll do a number on your fuel mileage, but it won’t hurt anything as long as you keep it to a reasonable speed. Honestly, though, I’m not sure how effective that’ll be in prolonging the life of the transmission. How loud is the whine?

It does sound like a planetary problem if it’s not happening in drive, because drive is a 1:1 gear and the planetary set is not “active” (I’m sure there’s a better term for that…)

A joke. You only need the planetary gearset when you’re in orbit. You don’t need it in hyperspace. Get clearance to land before you get to orbit. Docking fees are cheap these days.