I would jack up each wheel on a trolley jack and spin it by hand, I reckon you will find a tyre is bulged out of shape (wires letting go inside), may not be obvious just looking at it parked but you will soon see if it is not round when you spin it - easiest is if you get somebody else to spin it for you (at about 1 revolution per second) and you look at it end-on i.e. from directly behind or in front of the tyre.
Otherwise perhaps a bent wheel, can't think of too much else that would make a vehicle do that.
Needless to say if a tyre is doing that, change it immediately, as it could go boom any time!