You only hear it when the car is moving?
Does the sound stop or change while altering direction (usually left OR right not both)?
Got Steel Wheels?
I had a Kia Cerato that developed a similar sound. A high pitched creaking sound, a coarse squeak. Kind of like that noise you get on a very dry, very tight, nut (like a wheel nut) when you apply strong tightening force (sound about right?).
Drove me nuts for weeks. I swore it was a brake or bearing problem.
Turned out to be a very fine fracture (crack) in the rim, near the wheel nuts.
The reason I think it's worth a mention, it happened to two of the rims on that car 6 months apart... not a one off.
The rims on the Kia and Hyundai (Elantra) are similar (I bought a second hand Elantra Rim to fix the problem!) so it is possible they are sourced from the same factory !!!!!
Sounds identical to me... intermittent, outside, and got less severe the faster we went.
Worth checking out IF you have steel wheels (alloys won't make the sound!)!
Take the wheel covers off and look for a very fine black line (hard to see - the brake dust may have stuck to it though) somewhere, more likeley to run radially (ish).
From your description, and the fact that you say the brakes have already been checked (it is so hard to see, it could easily be missed in a wheel check)... I would ALMOST put money on it!