Small Engines (Lawn Mowers, etc.): TORO self propelled,Model20067; S/N270020231 Mower, transaxle, gearbox

Question
ok if just push the mower; when I use self-propel, very LOUD noise.  (maybe gearbox???) any simple way to discover if it is gearbox problem?
Thank you very much for your answer and consideration.  

Answer
If it's the gearbox, it should be evident by turning the wheels by hand with the engine off.  It could be the gearbox, which requires replacing the entire transaxle, one of the drive gears at the wheels and maybe the drive belt rubbing something.  Often the gearbox will fill with dirt and grass and make a lot of noise.  Usually at that point the transaxle will need replacement.