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 2009 Mercury Mariner problems
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 transmission oil cooler may develop a leak, which will require replacement to repair.
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.