The tires make the noise
Overall vehicle noise consists mainly of tire noise and the noise level increases with speed
Purdue University (2005)
This report from the Purdue University in Indiana establishes the relationship between the speed of motor vehicles and the noise level. The graph (P.4) below is an extract which illustrates this relationship.
It also points out that it is mainly the friction of the tires that is at issue, not so much the noise of the engine. In fact, total vehicle noise is mainly made up of tire noise and the noise level increases with speed. From this graph, we can deduce that reducing from 100 to 70 k / h would reduce the sound level by at least 4 to 5 dBA.
WHO – Noise guidelines (1999)
WHO – Noise Guidelines (1999) « The noise of road vehicules is mainly generated from the engine and from frictional contact between the vehicule and the ground and air. In general, road-contact noise exceeds engine noise at speeds higher than 60 km/h. » P.26