replaced lower unit still have noise

I found a guy who had 2 lower units and he let me take them home to try before paying. They both match my 1998 200 EFI Mercury. I changed gear oil and water impeller on both tried both of them on my boat. I still have some noise and vibration. Could the female drive tube be missing a gear up top? Or what ever drives tube have an issue> Kind of lost know that 3 lower units make the same noise, 2 have less noise than my original. Is this normal? Won't get to put it in water to see if its better until Wednesday at earliest. Any suggestions would be greatly appreciated.