I would inspect the brakes. My car has that whistle you describe in the left front, I ordered the new pads and will be installing them this weekend. If it doesn't cure the problem I'll look into replacing the discs.
It may just need some brake pad paste for you though.
After inspection of my front pads, I decided even though the sensors are not making the light come on in the car, the pads looked very low, and in comparison to the new pads, They are lower than I would like, so out they come.
So this sunday I'll be doing it, and I bought extra paste just in case.