wordless hum

The following is taken fron The Economist. 'hum' means make a low sound,but what is "wordless hum"? I am very puzzled.
"ONCE the soundtrack to a financial meltdown was the yelling of traders on the floor of a financial exchange. Now it is more likely to be the wordless hum of servers in data centres, as algorithms try to match buyers with sellers. But every big sell-off is gripped by the same rampant, visceral fear. The urge to sell overwhelms the advice to stand firm."

It's true that 'humming' never has words because it is just making a tune with your mouth closed.

However, I think the author wants to emphasise the difference between the 'yelling of traders' and the non-human sound of a computer humming, so he's added wordless because computers can't 'yell' words, like humans.