There have been some good responses here. Reading the one about the BK117 "mobile phone" reminded me that the Radar Warning Receiver (RWR) on the Chinook has both a monotone and a "trimphone" warble - the duration of each "blast" signifies a different kind of warning. It might be worth highlighting that in the early days of RWRs (60s Vietnam), fixed-wing pilots were subjected to so many audio warnings (RWR, their own Sidewinder missiles "growling", voices over intercom and R/T) that they were unable to filter out the essential from the background clutter. The warning must alert the pilot clearly and accurately, but it must not prevent him from being able to take effective action. But then you know all that already, eh?
Cheers