given that its quiet in 4th tells me you have a bearing issue. you could pull the trans and re-shim the input spline bearings and tighten the clearence over the whole main shaft. that wat you don't have to tear apart the whole trans, its a bit tricky because you don't want to remove the front shaft, just the holder . then add about .015 shim and see how much end play you have over the length of the shaft(add or remove as needed). if you can get it down to .002 you will be good.
before I forget, if you find a bad bearing don't forget to replace it while your there