Is the grease old? Crusty? I'd start there; clean and flush, check action, relube and check again. Old grease gets hard and crusty and just a tiny bit can do it. (or a spec of sand, etc..)
Look at hubs and close quarters for scratching of wheels and axles.
If cleaned and flushed, the noise is going to make "dirt" before other suspect areas.
Motor type? (can?)
Axle "end play" left to right? There is a sliding relationship with the wheelbase of 3+ axles that allows tight curves... that's why some use blind drivers; a flanged wheel would need too much end play. But worm drives being centered and not sliding helps their efficiency.
or runout play (oblong axle bearings)
Shimming for closer tolerances isn't unheard of at all. For axles; consider a smooth E clip. Easy on/ easy off; no wheel pulling or re-gauging of wheels. They stay put