---

If its a new mercury 4 stroke you are correct i hade the same problem with my 60hp took it to the dealer and they said it was the throttle body and ordered a new one (LOL) so i whent home and fixed it myself.
It squeals from the top of the drive shaft where it enters the bottom plate the powere head is mounted to. There is a small metal bushing thast it passes through and that needs lubricating. You need to remove the bottom end and drive shaft as if you are going to change the water pump grease the bushing and shaft and re-install use high presure grease like water proof wheel grease for your trailer.