Could be any number of things. Spring and hangers, steering related, kingpins and bushings, or even brake related. My bus has rear air ride and will leak down after sitting a few minutes, producing some unsettling popping, "groaning" and hissing sounds as it does (but otherwise harmless). I will eventually replace the air bags and leveling valve. I've driven large vehicles that made occasional suspension sounds for months with no apparent ill effect. However, I did inspect for any obvious safety problems (and found nothing noteworthy).
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