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.Google+
Car Problem Reports
Mercury Mariner Knock/Pop Noise from the Steering Column when Turning
Mercury Mariner Problem
Average Mileage: 74,310 mi (54,500 mi - 101,000 mi)
Visitor, 2008 Mercury Mariner
When making a sharp turn my steering wheel made this popping noise..it pop once..the steering was fine but I am worried that it my be a serious issue
Visitor, 2008 Mercury Mariner, 58,000 mi
Replace steering shaft and u joint. Known issue with Ford as they have newer part numbers that supersede the old ones.
Visitor, 2008 Mercury Mariner, 54,500 mi
Haven't fixed this problem yet. Just started yesterday. Just purchased this car 5 weeks ago used.
Visitor, 2008 Mercury Mariner, 88,365 mi
Found the U-joint bad, took it to the dealer for repair. Ford has to have more Quality and Reliability to their cars. Repairs today are costly in this economy.