2008 cobalt has recently developed shimmy in front end at low speed..<45mph is problem alignment or something more serious??also, road noise which has always been somewhat loud has definitely increased.

Your cobalt should still be under warranty, so you should bring it back to the dealer. If you have done massive driving and your out of warranty, then il'd have to say that you may have a wheel bearing hub gone bad and needs to be replaced. Check the tire pressure first, tire condition and look for any signs if tire damage like tire belt separation. Tire belt separation is when the steel belt in the tires that keep the tire it's shape has broke and there is a flat spot causing the wheel shimmy. If your not sure,rotate the wheel by placing the front wheels to the rear and the rear to the front, making sure the tires are properly inflated to the recommended PSI. Good luck and start with checking the tire first. 

Check your rims. Mine were bent and it was due to a bad strut and swar bar link. GM WONT address this issue but they are using substandard material alloy for their rims in my opinion. When I replaced the rims (used 4 rims $470.00) vibration went away. Then proceded to repair the front suspension. Sucks but it's true!!