There are several know issues with MPPSK exhaust rattles. First off the valves tend to rattle a bit especially during startup but after about 90 seconds that goes away when I always open them.
There is also an alignment issue where a section of the exhaust pipe can make contact with the car, a simple fix for that. I've seen mention of Cat's rattling on here but this is the first full replacement of the MPPSK exhaust I've heard of. Keep us posted.
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'16 340xi Mineral Grey/Black Leather, MPSS on 18" 398 Orbit Grey wheels, ZF 8HP50, MPPSK+CF Tips, ZTR, ZMP, ZTP, ZCW, ZDA, ZDB, Black Kidney Grilles.
'09 Outback for a beater and grocery getter.