What's this ?
 

On my SRT-6: I have a big black plastic “drum” thing bolted on the top of the wing (under the bonnet / hood) which has a Pipe coming out the bottom and a plug in the back of it.

I have three questions:-

1) What is it ?
2) What does it do ?
3) Why does it make a high pitched whirring noise when I first start the engine that stops after about 30 seconds and should it do that ?

It looks a bit like one of the old "after market" alarm systems you fit yourself. If it helps, it has a Bosch label on it and it isn’t the brake servo, in case you were wondering.

Re: What's this ?
 

Sounds like you found the air injection system. It fires-up when you first start the car to force fresh air into the exhaust stream. The air mixes with the exhaust to do some kind of chemical magic that causes the catalytic converters to get up to operating temperature more quickly. To me it sounds a bit like a blow dryer.

Re: What's this ?
 

Yes, that definitely sounds like what I mean. What a weird idea, I suppose it works, or does something beneficial, or it wouldn't be fitted. It does sound like a hair dryer though... I must say that is the strangest device I have ever had on a car and bizarre that it actually does what it is supposed to do. Still don't understand the benefit... weird!