How clean is the truck? Been through any mud or watter holes lately?
I've found that mine has a very distinct smell when the headers get nice and hot... when it's dirty from being in mud or water. It's literally the dirt/mud baking on it.

Next guess would be, as mentioned, coolant. Are you losing any?

With 200+ Billion electrical parts, the world most complicated machine is inside your own skull.

Question Reality.
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'89 Rnr DLX "SR4.5", 32s w/ 5.29 locked f/r blah blah