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