You probably noticed a little more "edge" to the engine sound. Removing the box also improves air circulation, but it's probably not measurable.

BTW, one can use a plastic pipe cap and a hose clamp to plug the hole. Get one that fits inside the hose.

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Ed in San Jose '97 540i 6 speed aspensilber over aubergine leather. Build date 3/97. Golden Gate Chapter BMW CCA Nr 62319.