Some owners of 2014-2016 Cadillac CTS or XTS vehicles equipped with a 3.6L turbocharged engine may comment about a brief rattle or tick when revving the engine when the transmission is in park or neutral. The same rattle can sometimes be heard at the beginning or end of acceleration while in drive. This condition may be caused by loose converter bracket nuts or it may be a normal characteristic of the turbo wastegate.

Also, check the nuts where the bracket secures to the bellhousing. 
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Inspect the M8 nuts that secure the catalytic converters to the exhaust bracket. If the nuts are loose, it is necessary to replace them with new torque prevailing nuts P/N 11546441. If the nuts are found to be torqued correctly, do not replace the turbo assembly in an effort to eliminate this brief rattle.