You may have noticed the pink chair in the Camping post. We're ok/encourage Maklin to play anything - regardless of gender norms etc. But there's no denying his automatic interest in trucks.
If Bvrrrooommm is a word ... it was Mak's first. If it's a word, it was also part of his first 2-word sentence ("Bvrrrooommm Dadda").
Different pitches accompany different types of 'trucks' (i.e. trucks, cars, construction equipment, tractors, etc).
As an aside, it does not however, refer to airplanes. Airplanes, according to Maklin, make the same sound as fish (puckered fishy lips and all).