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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Most reported 2008 Mercury Mariner problems
The transmission oil cooler may develop a leak, which will require replacement to repair.
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.