Finally got around to trying to find the source of the rattling sound, after a co-worker of mine said the engine sounded like a snow-blower he used to have, lol! Even though the rattling noise goes away after a few minutes, it's still there at start-up, every time (no relation to engine temp).

So I popped the hood yesterday, removed the air cleaner, started the engine, and listened. The rattling sound was actually coming from that pump gizmo thing on the exhaust side, and was coming thru the thick black hose that I disconnected from the stock air cleaner lid. Once I put my hand over the end of the hose, the sound stopped. So I left the hose there, found a thick rubber plug to stuff in the end of it, and now the rattling sound is completely gone. Doesn't seem to affect the engine performance, so I'm leaving it in.

Now that the rattling sound is gone, another little problem has cropped up. It seems to diesel pretty frequently now when I shut the engine off. It'll just sit there and knock and knock and knock, till I either turn the key to start, and let it run for a few seconds, or I leave it off and just press the gas pedal to the floor, which of course results in a thick gasoline smell.

I thought I had the fuel/air screw adjusted right, and the idle was set right around 800 rpms. So I guess I'll have to recheck those things, if the dieseling doesn't go away.