If it's a consistent ratcheting it might be in the exhaust valve train and not cats. Use the hose trick to find where it's coming from. Any kind of loose baffle in the cats will change with temp changes if that's what it is.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)