Everything is 1 to 1 in third - having a noise in third doesn't make much sense. There is very little preload on the #1 bearing in third - only 0-5 psi pre-filling the OD piston. If that noise is faint then it gets much louder in 4th then the first bearing to suspect is the #1 - that's the one on the OD piston - and that is on the top of the overdrive unit when you pull it off the trans. Sometimes it sticks to the OD piston. That OD piston should have a thick steel shim and the #1 bearing. The next one to get loaded on the 3 - 4 shift is the #2 bearing, the #2 is under the sliding hub spring seat - fits between the seat and the planetary. The next is the #3 - it rests under the ORC cam in the OD unit. The other bearing that gets loaded on the 3-4 shift is the output shaft bearing - it's in the back of the overdrive unit. It gets 800# plus load in OD, the OD piston shoves everything back and the only thing that stops it is this bearing.

Look the planetary over real good - real fine pitting in the teeth will cause noise. Usually the sun gear gets this pitting well before the pinions do in the planetary.