Troubleshooting a Yamaha outboard motor's tilt and trim is fairly straightforward, usually involving a thorough visual inspection and a check of the hydraulic fluid level. Things to look for include any build-up of dirt and sand, as well as a lack of sufficient or appropriate grease. A screeching sound while raising or lowering the motor is very likely indicative of a need for grease on the trim rams.
It is best to use a marine grease specifically formulated for a saltwater environment. Application should cause the screeching noises to cease immediately.
It is also good practice, while applying the grease, to check trim rams for leaks. Hydraulic fluid is of a very low viscosity and is therefore prone to leaking out when possible, potentially causing the motor to become stuck in the tilt down position. Cleaning leaked fluid away and refilling the tank is a temporary fix but the motor should be professionally repaired when possible.
If the motor is stuck in a middle position and unable to move either up or down, it may indicate a problem with corrosion. As a quick fix in an emergency, this can sometimes be remedied by hitting the motor with a hard object. However, it should be professionally examined later. Sometimes, corrosion on the rams is less serious and can be cleared away by wiping.