I took my axle out and greased the hell out of my speed sensor and torqued everything again and rode it some and no sound yet. Hopefully that does the trick, i have 2 dual sport rides coming up.

So I have a new 2018 KLX250s and the whistling started while I was on a pretty rocky / dusty Colorado trail. Sounds like a kettle and also has a slight scraping sound eventually. It ended up being some grime / dirt next to the speedometer. We loosened up the axle and sprayed some air blast around the area. Then we used lithium grease spray on some string and ran it all around the axle piking up that dirt. Tightened everything back up, sprayed more grease onto it all and the sound has been gone for two days. Drove me nuts for weeks as I thought it was the pads and had checked them to no avail. Only took about 2 minutes to fix and Im assuming will need to do it again after dusty rides. This probably inst everyone's problem but I wanted to let y'all know how I solved mine as it is extremely irritating. Happy trails!

Same same
 

I have the same problem it's like LOUD wind noise or a grinding sound,process of elimination,
*i spun the wheel on the jack slight grinding sound
Possibly disc grinding.
*speed sensor noise clean and reassemble
*front fender wind noise
*fork guards
I'm thinking if possible remove each item one at a time to see where this LOUD wind noise/grinding is coming from good luck! (Keep riding 👍🏍)

I realize this is an old thread, but has anyone considered the noise they are hearing is from their knobby tires?
I know off-road tires can develop a weird frequency on asphalt.