1999 Elantra drive train grinding noises?

sounds like the transmission is bad, one good thing to try is revving engine in park sometimes the trans will grind that way too if so definitely bad trans. Also try putting car in drive and reverse hold brake as hard as you can and raise idle of engine if grinding is present bad trans, if not have you considered wheel bearings you can check those by lifting vehicle and i'm sure you know check for play also try putting vehicle in neutral and spin the wheel listen for noise in bearing and transmission. Hope this info was helpful.