My old Malibu did the same thing, so did my G6 I had before. Both were 3.6's. Dealer told me on both occasions it was normal.
Sounds to me like you have the common Malibu vibrations. Not really related to this thread. See the big sticky in Problems/Service....Hey I wanted to bump this post as I am having the same shudder issues. Mine is an 09 with 3.6l and 8k miles. The vibration is noticeably worse when the Defrost is on, but does exist with it off as well. The Main times I notice it is when coming into a parking spot or slowing while turning the steering wheel. It is almost bad enough to rattle the dash. Took it to the dealer and they tell me its normal. Well I cant imagine this being just normal. Also I noticed a shudder while driving around 60 and downshifting to around 3k rpms and giving it slight acceleration. It almost feels like a wheel out of balance but goes away when it upshifts into a higher gear.