S. R. Faruqi:

By not clarifying the 'caravan of the dawn' he has bestowed on the verse a beautiful universality. The harmony of the first line too presents a picture of the awakening of the caravan and the shared intention of the time of departure.

By ik shor hai can be meant that people are calling out repeatedly, 'We've gone!'; and also that the clamor and tumult itself acts as an announcement that 'We've gone, and you're lying there asleep!'

FWP:

SETS == DIALOGUE; EK; EXCLAMATION
MOTIFS
NAMES
TERMS

The 'caravan of the dawn' can indeed be a caravan that sets out at dawn. But SRF points out that the verse leaves some thoughtfully-contrived ambiguity here. For thanks to the versatile powers of the izafat, the 'caravan of the dawn' can also be a caravan that is itself the dawn (dawn imagined as a departing caravan); or a caravan that belongs to the dawn (maybe the trailing clouds that come to light with the dawn?); or a caravan that pertains to the dawn in some other way (the journey of one's lifetime will be over all too soon?).

As SRF observes, it's up to us to decide whether the clamor consists of the words in the second line as they are called out by the people of the caravan, or whether the clamor of the departing caravan itself acts as a wake-up call, so that it conveys the message given in the second line.

Note for grammar fans: Urdu has a tendency often to be a step further back toward the past than English. Here's a striking case in which it's two steps further back. Literally ham chale means 'we left' (in the perfect); but in such a context, in English we'd say 'we've left' (in the present perfect). In addition, in such a context we'd actually say, in English, 'we're leaving', or even 'we're about to leave', which accurately describes the speaker's situation; while in Urdu, idiomatically people would say 'we left' to emphasizes the imminence of departure.