Turbo whistle and hole in airfilter housing

[Phil her up]

I have a turbo whistle that seems louder when the engine is cold and under a little load. I have done a search and read a thread where someone had a whistle and ended up replacing the turbo and still got whistle. I expected to find a split hose or something but nothing obvious - any ideas?
Also, when I changed the air filter recently I noticed a pre formed hole in the air filter housing. The hole was open with nothing obvious that had fallen out of it (ie a hose or whatever). I just blanked it off with a plastic bung but what was the hole intended for?

[BUSHWHACKER]

All the Surf turbos whistle so you're o.k. The hole you blocked is for the air flow sensor for the ECU. It should be there somewhere it's got a black cable sleeving attatched to it.

[Phil her up]

All the Surf turbos whistle so you're o.k. The hole you blocked is for the air flow sensor for the ECU. It should be there somewhere it's got a black cable sleeving attatched to it.

THANKS! .......... you're right it was there somewhere and now it is back in
How does it sense air? it just looks like a solid piece of plastic (rocket shaped) on the end of a wire.

[BUSHWHACKER]

I think it's something to do with air temperature.

[Phil her up]

I think it's something to do with air temperature.

I wondered if somthing was missing from it but looking at it again there seems nothing is broken off so cheers mate - another little mystery solved.