does the sound change when you press the clutch pedal ?
if so then its quite likely that the clutch release bearing is faulty or of the wrong type, especially if the sound stops when you press the clutch
if this is the case you can get away with it for a while usually about 5K miles before its starting getting very noisy and the gearbox will have to come off again
if the noise is there all the time then I'd start by draining off the gearbox oil and strain it though some paper towel as it drains to catch any debris
OK if the noise is occurring under acceleration and only in 1st gear then its quite possible that the gear is damaged
so I'd say the next step is to drain the oil and inspect it for signs of metal debris
do you still need help?
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