What would cause a 'roaring' noise under the hood of a Mercury Mystique that sounds like a vacuum cleaner when the weather gets colder?
That is what is referred to as mooseing. There is a fix for it.
What is the fix?
On my car, it was the 'idle air control motor' mounted between the air cleaner and the fuel injection system. The new part was around $165, total bill with diagnostics was just over $300.