Possible Pinion Replacement

I doubt that it will be in the drive line, but it would not hurt to inspect uni's and pinion nuts are tight and there is no movement. This sound like a transmission or transfer noise. Best to get a mechanic to road test as listening to these noises are a far better way to start a diagnosis. Just make sure that the oil levels are okay and you do not have contaminants in the oil. If you have not done so in some time, change the oils and check oil oil for metal discoloration and shavings on the drain plug. Metal contamination in the oil can look like a silvery sheen in the oil.