I'll bet you it'* a spun bearing. If you red lined the car passing someone, then noticed the noise soon after, I couldn't imagine that it wouldn't have been a rod bearing.
If it'* a rod bearing:
In park at idle or any rpm in park, you should hear a steady, consistant knock-knock-knock-knock-knock when you rev the engine. It should be directly related to engine RPM.
When you put it in gear and brake touque the engine up to about 1500 rpm without moving, the noise should all but dissappear.
If you notice your oil pressure has changed makably lower now that you hear the noise, it has to be a spun bearing. This just happened to my friend'* '96 Bonneville a month ago. Terrable noise at idle with oil pressure touching the red, not so bad when up to speed.
If you keep driving your car like this, you're going to sieze the engine and you'll be stranded for sure.
I've also went through my own spun bearing problems, and my car behaved exactly as you describe. For some reason, 3800s just can't hold onto their rod bearings. It'* a shame such a good engine otherwise can't just stay together.