Rough idle in 99-up stangs??

Here's the deal. Helped my girlfriend pick out an 01 sixer. Noticed that at idle it didn't feel that smooth, and the same thing when I revved it up a little. A few weeks later I helped my friend to get a 00. Same thing with that one too. I have a 96 v6 and it doesn't do this. Any of you six fiends know anything about this??? Any help would be appreciated.

It seems fine when it's idling and while you drive, it just does it when you rev it or touch the gas while it's parked. It's not too bad, just noticeable. Not very smooth sounding or feeling like it should be.