My BMW has 106,000 miles on it. The check engine light came on. The spark plugs are original and are rated for 100,000. I am experiencing poor acceleration, it's like I am driving a car with a turbo. It starts off slow and then all of a sudden it goes like the wind. There is a slight rattle under the hood when I am taking off, but not when I'm driving.
You will want to have the catalytic converters checked. The 540’s are notorious for the catalyst inside the catalytic converters breaking apart from the outer wall of the assembly and rattling around, especially upon take off. This will also create increased exhaust back pressure which could give you that lagging feeling on acceleration. You may want to enlist the help of a mechanic, such as one from YourMechanic, who will have the tools and expertise to diagnose and repair the acceleration and rattling issue.