Which of the following physical phenomena indicates an imminent BLEVE (Boiling Liquid Expanding Vapor Explosion) on a pressure tanker exposed to fire, signaling an immediate full-speed withdrawal of all personnel?

1. A rising, high-pitched whistling sound from the pressure relief valve and visible bulging or discoloration of the upper tank shell
2. The vehicle radio playing classical violin music
3. The tires becoming slightly dusty from road soot
4. A sudden drop in the vehicle's windshield wiper fluid level