Why does the front of my car shimmy and sound like a helicopter at slow speeds?

There could be several things going on: 1: Did you have the front end aligned? Alignment can cause a front end shimmy. 2: Did you have the front rotors machined as well? Rotors can warp and cause the problem you discussed. 3: Have you rotated the tires previously? Front wheel drive cars have a propensity for "cupping". This is where the inside tire wear is uneven but not necessarily noticeable as excessive wear. If they were rotated previously and then put back in their original location then you would not have noticed till the tires were rotated again into thier original position