Mmmm.

Try again .... there must be one of the 8 sensors not clicking. Make sure that when you're listening that it's not the next sensor you're hearing and you're mistaking it for the one you've got your ear to. You said one was quieter than the others so pay particular attention to that one.

Also get your fingers in behind the bumper and just make sure the connector is firmly in place for each sensor. I've personally found that road bumps etc. can disturb the seating of the connectors and knock the sensor out.

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2001 X5 4.4i