Tires: tire noise/searching, michelin primacy, alignment problem

Question
Thanks for taking my question...
I just purchased a 2009 Prius with the touring option, which came with Bridgestone Turanza(el400)195/55/16. I just changed to Michelin Primacy mxv4 205/55/16. So far it seems to have better handling. However, there is more noise on less than the smoother freeway surfaces and a lot more thump on strips and patches. And also,(this is new) the front end searches at highway speeds. If I take my hands off the steering wheel for a second or two, the front will be all over the place. Not a steady straight line. Is this due to a deeper tread, softer rubber...I am constantly correcting the steering direction. Very unnerving. They are inflated to factory specs, 35/33psi. It wasn't doing this with the Bridgestones so haven't thought about an alignment problem. New tires have about 300 miles on them.

Answer
Dennis,

Is the only time this happens is when the pavement has grooves in it?  

If so, you have "Groove Wander".  Some folks call this "tramlining".  This is where the grooves in the tire want to follow the grooves in the pavement.  There's no fix other than to replace the tires with something else.

If not, then I think you need to look at the alignment.  I'll bet the additional width has caused the center of the contact patch to be larger and that is causing the suspension to react more.  Toe in is what I suspect is out.