On a quiet road, in 4th gear. From 3,500 to 4,000 RPM the motor sounds like a jet, it has a whir about it. I enter and exit the highways just to hear that sound.

 

I get that in all my cars, Honda, Toyota, Audi... It's a resonance of some sort, either transmission, engine, or both, and totally normal. There's also a distinctive intake resonance at higher rpm, corresponding to the max torque output of the engine - around 4800 for the Fit I think.

 

Now that you mention it, I do notice that, too, at around the same range - even on my smaller engine with no VTEC and half the valve count.

Unlike your L15As, though, my L13A3 makes peak torque at 2800 RPM.