Well, there are several possibilities. Could be that whatever was in the trans had an additive that was keeping it quiet, or being overfull could have kept it quiet. I would not overfill it again, as that only invites oil aeration and overheating. It could also be that the additive package of the fresh, correct oil has removed the deposits or varnish left by the old oil and it's now making noise. In any case, if it's the correct oil in the correct amount, you've done nearly all you can. Give it some time for the additives in the new oil to work into the trans internals and see if the noise doesn't improve. Otherwise, you can try different company's versions of synchromesh and see if the noise doesn't go away.
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