in gold on a red firetruck (lines 4-6)
The number and the fire truck are the two stars of the poem. The number is mysterious and painted in gold, a color symbolic of richness and traditional majesty. The fire truck is totally commonplace and painted bright red, a primary color. In the modern world, the majestic and the commonplace exist side by side.
tense unheeded (lines 8-9)
The scene is full of excitement and danger ("tension"), like you feel when you watch a scary movie and know that one of the characters is about to get knocked off. But, except for the speaker, the noisy ambulance goes "unheeded." Modern life requires that we become desensitized to some degree to such excitement.
to gong clangs siren howls (lines 10-11)
The clanging and howling are noisy sounds, not meant to be beautiful. (Their sole purpose is to warn people to stay the heck out of the way!) But the poem suggests that they may have a certain beauty of their own, if you perceive them the right way.