When we meet the Whos, we learn a few things about them that the original Boris Karloff narration of the 1966 Grinch cartoon left out. The first thing is that they're microscopic. The entire species lives on a speck of dust on top of a flower. The Whos are like a hippie version of that tiny Kryptonian city that Superman keeps around, presumably so that he can still say "But look at how BIG and MIGHTY I am!" whenever he gets his ass kicked by Batman.
The other thing is that Whos never make a peep.
Whoville: Before the catastrophe.
The Whos, at this point in their history, don't blow floofloovers or bang tartookas, and don't play noisy games like Zozittacarzay. Instead, they ride bikes. They go on walks. They're peaceful and quiet; so much so that nobody but Horton even notices they're there.
When Horton does notice them, it's a little bit weirder than you might realize. Because Horton doesn't hear a speck of dust bustling with noise -- he hears one Who yell out "Help!" and nothing else coming from all of Whoville. In fact, the town is so quiet that Horton thinks there's only one person there at first.