Slight fishtail/waggle at 18mph

[algiorda]

My box seems to be fishtailing a bit. I thought it was my imagination, but I mounted my smartphone on a magnetic holder and at around 18mph, it seems to oscillate like the car has a slight waggle or what I call fish tailing.

Where should I check first? Tires (balance), rotors (warping), alignment (God knows)? At speed, the car does not vibrate and feels smooth (imperceptible). It is also smooth with no movement at Idle. The waggle seems to increase with speed.