One obvious factor is that your old tires may have been pretty smooth, and smoother tires will be quieter.
Tire noise depends on the tread pattern, the roughness of the road and the roundness of the tires. Really, tires can be out-of-round, and the high spots will thump on the road during every revolution.
But assuming that your tires are more round than square and that you’re on a relatively smooth highway (that is, you’re not in the state of Pennsylvania), the real question is why rolling rubber should make any noise at all; you’d think there would be nothing quieter. And indeed, tire manufacturers put a lot of effort into making their products operate as quietly as possible. Here are some of the things they have to consider.
As you can guess from the complexity of the sound, it’s hardly a pure musical tone, it comes from a combination of several factors that make the tires vibrate. When the walls of a tire vibrate, it makes the air inside and outside vibrate also, and that’s exactly what sound is: vibrations of air.
The source of most of the vibrations is the “contact patch”, the constantly changing flattened area that is in contact with the road. As each segment of the tire comes around in turn, it slaps against the road and is flattened into the contact patch. That constant slapping makes noise. But your new tires aren’t perfectly smooth (unless you’re a race driver). They have crosswise tread grooves that divide them into separate blocks of rubber, and those blocks hit the road in a rat-a-tat-tat machine-gun sequence.
More noise. Moreover, as each block of rubber reaches the back end of the contact patch it snaps back into shape, again making the air around it vibrate. Still more noise.
A less obvious source of noise involves the escape of compressed air. As the tire turns, a groove entering the contact patch can trap some air and compress it against the road. Then when the groove leaves the contact patch the trapped air is released to the rear with a sudden, if you’ll excuse me, fart. Experiments have been done with porous road surfaces that can significantly reduce this source of noise by allowing the air to bleed off directly into the road.
All of these effects depend on the groove pattern in your tires and the characteristics of the road surface. And the faster you go, of course, the more times per second all of these noise-producing processes are taking place. Driving at slower speeds will not only reduce the noise coming from your tires but will completely eliminate that annoying siren.