My car has always had a y pipe with a flexi, and the driver side second cat (the unit has two cats) is loose and rattled at certain revs or when I turned the engine off. I can hold the unit and tilt it one way or the other and you can hear/feel the core slide inside. I had planned to drill a small pilot hole and screw a small stainless self tapper from the underside to hold the core steady which would sort my problem.