Only other thing I can think of is your oil pickup screen is clogged. I agree, given those white dots that someone's been working on her.

The reason I say this is that in our extreme cold, using 5W30 oil, is that if there isn't an oil pan heater plugged in, during the first few seconds after start up the engine will rattle, as the oil is so sluggish that it's not getting picked up right way, and your oil light comes on dimly.

It's a bit far fetched, but I'd sure go do some digging around and find out what maintenance was done to that truck before you bought it.