I've had failed cats. Bad cats rattle. A tapping sound is something else. I suspect something is loose too.

Mine had lifter noise, so did my '21 HEMI at start up. LiquiMoly Ceratec additive stopped the ticking in both. One can will last 30k miles. Both vehicles are close to their next oil change. I'll see if the ticking stays away. So far, I'm happy with results.

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'06 X5 3.0i - bought @143,123 miles (12/26/20)