If the transmission oil leaks out of left side half shaft area, it may be from a worn axle shaft seal. The seal can be damaged by a poor surface finish on the axle shaft. The axle shaft and seal should be replaced to correct the problem.
Car problem reports
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Most reported Mercury Mariner problems
The transmission oil cooler may develop a leak, which will require replacement to repair.
Looseness in the steering shaft and steering shaft coupling U-joint can cause a knocking / popping noise from the steering column when turning. The steering shaft and coupling U-joint will need to be replaced.
The check engine light may come on due to failure of the Variable Camshaft Timing (VCT) solenoids. The VCT solenoids will need to be replaced if they are the reason the check engine light came on.
A defective power steering return line can cause a knocking noise when turning or going over bumps, especially when the engine has reached normal operating temperature. The power steering return line should be replaced.
A whining from the front while driving may be caused by the front axle shafts. This would be felt between 20 and 45 MPH, and the half shafts can be replaced with revised dampened axles.
Shock bushing failure in the rear can allow the shock absorber to contact the body and cause a squeaking noise over bumps in the rear. The faulty rear shock absorber bushings should be replaced.