> I did a mix and match with the 3 transfer gears as one was missing on the
> gearbox I put in could this cause it?
Yes.
The gears mate together and run quietly that way.
If you make up a set from used gears, you are likely to get noise.
I put a transfer gear back in the wrong way round once on an early box. It
sounded like a set of straight cuts.
If the noise is only slight, I would leave it alone. Changing the oil more
frequently might be a good idea as well.
John