Chevrolet: 2004 Chevy Malibu Classic -shuddering in Rev!!!

My son just purchased a really clean 2004 Chevy Malibu Classic with
less then 40K miles on it. Interior and exterior are mint.
Everything clean and CarFax report clean. He test drove it and all
seemed fine. Then after he brought it home he noticed that if he was
at a stand still and shifted it into reverse and back a short distance
and then stopped, leaving it in reverse, the car shudder quite a bit.
No problems with response or handling, just that suddering. He also
noted that if he was in drive and sitting still and turned the AC on,
he said it shuddered just a little when it first came on, too.

He's taking it in to have it checked out but we felt it would be good
to get some 'second opinions' on something like this before he went to
be a little more knowledgable when he was speaking with the service
folks.

Thanks in advance for your assistance!

"infiniteMPG" <com> wrote in message
news:googlegroups.com... 
Pretty likely there is a broken, worn-out, or loose motor mount or exhaust
hanger. If you try to back it up while watching the motor you may be able to
see the motor move. Some movement is normal and it is hard to describe how
much is abnormal.

Don Young