Rusted brake rotor
Sounds like you have rusted brake rotor. You can either rotate/polish the disk for $25/rotor, or get a new rotor for $30/rotor. Sport rotor cost $50/rotor.
If you buy from dealer, they charge $50/rotor for normal model. You can change yourself, but if you need rotate, then some workshop has the tool.
Logically, you should experience steering shake in certain speed, or at 120 km/h
I recommend to change sport rotor. Better performance, quality, and strong brake