Taken from Unofficial E36, Debaffled Airbox
The second "baffle" is on the downstream side of the box from where your filter sits. It looks like a large diameter tube molded into the plastic leading to the hose that attaches to the throttle body. I originally took this out, but then put it back several months later. The reason why is I got to examine a Dinan AirBox. The Dinan configuration is very similar to the stock box, but without the intake silencer we just removed. The second "baffle" in the box is retained in the Dinan design. I started thinking about this, and concluded that the second "baffle" really doesn't seem to offer a significant restriction anyway, and besides it might be some sort of "velocity stack" for the air flow. So I put the thing back. But if you want to remove it, here's how.
***Anyone mess with the second baffle on their car? and input about it. I'm just curious.