Helen,
Dogs usually howl like you described, because the sound hurts their ears.
Shrill noises like sirens, etc., is like a piercing noise, and dogs have very sensitive ears. If you would speak to them in a very high pitched tone and very loudly, they would do the same thing. They would start howling. Sometimes when people run appliances like sweepers, or machines that make a loud whining noise, they will howl also.
Unless these are not new noises to them, and they are just now starting to howl at them, then I would consider this unusual behavior.
I heard several dogs whining and howling off in the distance, around 1:00 a.m. on Friday night, June 3. I did not hear anything myself, but whatever they were howling about, was probably closer to them, and that got them upset.
Susan