I have a '91 Buick Century, 3.3L V6, 170k miles: it started making a metallic knocking sound under the hood; but only really noticeable while idling. When I give it a bit of gas in Park or start driving, the sound goes away. Looking under the hood while idling the sound is disturbingly loud, and seems to be coming from the lower engine. All fluids are OK, I don't see anything loose. Any ideas ?

You probably have a bearing going on a connecting rod. If the knock gets louder as you increase the load on the engine, than its connecting rod.

Either way, your due for a rebuild. Nothing will prevent the inevitable. No special additive, no rain dance.

Funny thing about those 3300 engines......brand new, they has a rough idle.....just step on the gas and elevate the rpms slightly, and it would go away.....so people would adjust the throttle plate to eliminate this, but it caused other problems like hard engagements when shifting from Park into reverse or drive.....

That engine was a bear though on the highway......lots of power.....and if I remember right, I don't think it had an EGR, the way it was designed? Maybe I'm wrong?

Now the old 3.3 of the mid 80'*, that was a piece of crap, and "fun" to work on....

Thanks for the replies, Tech II and Mike1995. My problem sounds more like that described by Tech II; I only hear the knocking at idle - when I increase rpm'* the sound goes away. At idle it'* very noticeable. I'm stumped. And yes, this blue beast has never been lacking in power; not great mpg, but I can always pass anyone I want...no problem.