Tonight I noticed a knock in my 98 TJ that I can only hear at idle. It's a 4.0l auto. I've had this jeep for about a month and this is the first time I've noticed this. It's only at idle, if I give it a little gas it goes away, and I can't hear it at all while driving. It doesn't make any difference whether it's in gear or not. I read some posts on another forum that said this was a common problem, and most said it was nothing to worry about and that it was "normal" but it was only XJ owners. Has anybody else heard of this?