Like paco says, it was the Bell 47 sound that was most often used in the past. More alarmingly for the informed was the way in which a helicopter lifting into the hover and departing was accompanied by the sound of a Bell 47 winding up, while a helicopter landing would be the same machine winding down...... Never saw any signs of coning up though!!
If you asked a non-pilot type, most of them seem to think that it is perfectly normal.