[outback 1970]

Earlier this summer the transfer case was makeing grinding noises in all gears so I removed the transfer case dissassembled it and found a broken snap ring in the tail houseing I replaced and examined the snap ring that was bad and looked over all the gears I thought pretty well I put it back together everything worked great up untill last night I locked in the 4wd and it was working fine then all of a sudden it started makeing a intermittent grinding noise from the t-case and it only does it in 4wd? any advise would be appreciated