Well, so much for "bulletproof" straight 6, motors knocking :(

[BIG CTY]

I'm going to expect the worst, but hope for the best here. I was driving the balls off of my rust bucket awhile ago and when I came to a stop, I heard knock knock knock going along with the idle. When I speed up, the knocking gets quicker, when I slow down it gets slower. Its not nearly as loud at idle as it is when I get in gear and go. I was running a good 75-80 for about 15 mins, RPMS were between 3500 and 4k.

Now I know enough to be dangerous, but I'm not sure where to start here. I'm kinda hoping that its something simple, but anytime I hear knocking in the engine area I get scared. Anyone have any advice? The truck is for sale and there is no way I can sell it sounding like this. Where in the he!! do I begin tracking this down?