It's the frequency that two other frequencies create, typically it's a low frequency.
Like if 1 tire is making a 1000 hz sound and the other is making a 1100 hz sound the resulting heterodyne frequency would be 100 hz.
As a test, take one rear tire and move it to the front.
Any change in sound?
Rotate the rear tires side to side (now rotating in the opposite direction)
What brand of tire?
If you look at BFG TA Radials you will notice that the treads are not uniform size and they are in a random size spacing. Like large, small, medium, small, large, small, medium, large etc.
The random size pattern reduces the likely hood of a tone being generated
Like when you are on a highway and it has left to right cuts in the road bed, you here a tone, if they are all equally spaced the time is constant. But if the spacing is specifically spaced it plays a song.