I am changing the air filter to a BMC and doing the cold air intake thing at the same time (a'la Fabspeed red silicon hose).

What I have discovered is this small filter in the bottom of the filter box (see picture) and I am wondering what the purpose of it is?

And also it might be small, but I guess not to small to not suck in hot air from the engine compartment?

 

I believe it prevents water ingress.
Be very careful to situate the CF heat shield appropriately otherwise it will rub and cause problems on the A/C pipe and or cam wheel

 

Best place to source the BMC filter to replace the OEM filters and maintain the OEM air box ?

 

Hi Molar.

Yeah that might be, but actually there is a drilled hole in the lowest point of the box (for draining water I guess) and this filter is situated quite a bit higher than that.

I guess there is no heat shield (it's a 2005 C2S).

I am keeping the stock air box -just changing to BMC filter, blocking the resonate chamber and fitting silicon hose a'la Fabspeed as mentioned.