It is surprisingly difficult to find an English source for this.

[1] shows on page 3 a significantly higher tire noise for 50 kph (+8 dbA) but doesn't show lower speeds than 50 kph.

[2] is in German and from the Austrian Federal Environment Agency and states that for cars build after 1996 25 kph is the point where the tire noise takes over at a steady speed and at 50 kph when the car is accelerating.

1: http://www.asphaltroads.org/assets/_control/content/files/an...

2: http://www.umweltbundesamt.at/umweltsituation/laerm/laermsch...