Engine noise at idle.
 

I have searched the forms about it, but I can't find anything conclusive. Anyway, here is the issue, Ever since I bought my Dakota (1999 3.9 5spd reg. cab), about a year ago I believe, I have noticed a knocking sound. It seems like it only happens at idle and I can't hear it unless I am parked next to something and have my window down. I't doesn't get louder as I increase RPM(in fact it seems to go way). I've always just chalked it up to being able to hear more engine noise since it is a manual transmission and doesn't absorb as much NVH as an automatic does with its torque converter. But tonight my curiosity got the best of me and I decided to search the forms about it and well the results came up with things from spun main bearings to sludge that can cause the noise. So my question is, Is this a normal noise ? It doesn't seem like it. What do you guys think it might be?

Edit: I haven't noticed any power loss or anything, it runs as strong as ever.

After listening a little closer, I can just barely hear the sound while inside my truck, with the windows up, and only when I'm stopped.

maybe timing chain slap?
 

My 3.9 with 120k on it developed timing chain slapping the cover when the engine is not under load (i.e. at idle). It's audible. I've had two mechanics listen to it, and both said not to bother with it unless we need to go in there for something else. Apparently some of these engines do not have a tensioner in there, and when then chain inevitably wears and stretches, it starts slapping the cover a little.

This may not be your issue of course, but you could have it checked out.

My Dak is a '97, so yours may have a tensioner in it for all I know. I believe you can install a tensioner when you replace the chain on my model year.

That is a possibility. I guess I will take to a mechanic when i get the chance. Or I could have one of the automotive engine performance teachers here at UNOH take a look at it.
Thank you for your input :)

It could also be the lifters. The Dakota i had had a lot of lifter tick even when i put a new ones in.

3.9's are known for timing chain slap especially the 97 and older ones. usually its not a huge deal and people just leave it be until they go to do the water pump. bad thing about it slapping tho is it will break the oil dripper off that drips oil on the chain to keep it lubed.

I guess we'll have to wait and see until i get enough money to replace the timing chain. thank you for your input guys