Your engine has a viscous fan clutch that tightens based on the temperature of the air exiting the radiator. In hot weather stop and go driving this occurs when the engine temp hits about 100C, and when the engine revs up you will hear fan roar.

If the engine temp reaches about 105C the electric pusher fan(s) will engage at high speed and make a "whirring/roaring" noise that you will probably hear, even at idle speed.

Duke