It is simply impossible to not hear that sound, and Obi has been hearing it without being able to identify where it comes from for a while. He's curious, extremely curious. And then when he does find out where the sound comes from, he's even more intrigued because while he has seen those peculiar machines in the Near Shore, he has no idea what they actually are. He's assuming that they're not cars, but he's also assuming that cars requires four wheels exactly, no more, no less.

She passed by to fast another time, but when he does see her approach in the distance as he's sitting in a bare tree, he swings down, dangling upside down with no effort, and he waves. Possibly too much but, he's not the number one serious individual in the Far shore in the first place. If it doesn't catch her attention now, he supposes that he'll have to find her when she's not straddling that machine at high speeds.