Lines 7-13 Summary

Get out the microscope, because we’re going through this poem line-by-line.

moving

Again, every word in this poem adds new information, as the scene builds piece by piece. The line breaks create time to digest the language.
The poem slows down in the middle, as if to watch the fire truck pass.
We did not know that the truck was moving until now. Is it whooshing past the speaker, or has it already overtaken him?
Here's an interesting question: what is the subject of the verb "moving"? Is it the fire truck or the figure 5?

tense
unheeded

These are the most "figurative" lines in the poem; that is, the language here cannot be taken literally.
Williams continues the slow-motion effect of one-word lines.
The question we asked about line 7 – whether it was the figure or the fire truck moving – becomes even more important here. Is it the figure 5 or the truck that is "tense" and "unheeded"?
Try flexing all your muscles at once and walking around the room. That's one example of "tense" movement.
Of course, "tense" does not describe a kind of movement so much as the general atmosphere, or perhaps even the speaker. Think of how your body tenses up when a loud fire truck go by. Everyone is trying to stay out of its way, and because fire trucks do not have to obey normal traffic laws, they always seem slightly out of control.
"Unheeded" means "neglected" or "ignored." No one is paying attention to whatever it is that is moving (either the number or the fire truck). At this point, we think it makes more sense for the number to be "unheeded," because it is such a small part of the truck. Few people would notice such a number, but everyone would notice the truck. But you can make up your own mind about that.

to gong clangs
siren howls

These lines give an idea of why a part of the fire truck would be "unheeded." There is so much noise and commotion coming from the truck that it's hard to pay attention to anything else.
The sounds completely overwhelm the visual aspects of the scene.
The "gong" refers to the large bell fixed to the fire truck. (Nowadays, fire trucks have electronic sirens and horns, but in Williams's time there would have been a physical bell.)
But a "gong" is also an Asian musical instrument that you hit with a mallet to make a percussive sound. (You might associate them with bad kung fu movies.) Williams was very interested in Asian art and religion, and this poem resembles forms of Asian poetry like the haiku.
In addition to the gong, the truck has a siren that "howls." It's as if the poem has been silent this whole time, and suddenly Williams has turned off the "mute" button to unleash a cacophony of sound.
The word "to" suggests that all these sounds are like a musical accompaniment to the movement of the truck.

and wheels rumbling
through the dark city.

Line 12 continues to emphasize how much noise the fire truck is making and why this noise might distract normal people – people who are not as perceptive as the speaker – from the "great figure."
The ominous word "rumbling" conveys a sense of the large size of the truck and also suggests thunder. It's as if the lights and sound of the man-made truck have taken over the role of thunder and lightning in the rainstorm.
In the final line, the scene returns to darkness. You might be surprised by the description of the "dark city" after the mention of "lights" in line 2.
Williams finally confirms that we are in a city.
Notice that the lines get longer again toward the end of the poem.
The scene seems surprisingly empty and lonely now that the fire truck has passed by.