I had noise in 4th and 5th too, but it turned out to be a leaky seal that had all but emptied the gear oil...filled it back up and the noise went away (aside from internal gear whine I had a rattling release fork when the fluid was low).
You may be in for a rebuild, if the transmission fluid is filled to the top of the fill plug. Try draining a bit of fluid and see if there are metal shavings in it. If there are, it'll look iridescent like metallic paint...
--Tim
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1990 5spd V6 Camry (Still kicking at 393,000km) 1991 Celica GTS -- Pappa needs a 3SGTE...and AWD for all this friggin' snow
Honda my A$$, you just can't kill a Yota...