Having an interesting issue with the pick up. Started it today, was cold exhaust sounded kind of loping and choppy. Drove to fuel station when warm sound gone now but oil pressure seems low at idle. Below 40 psi. 275k on it. Noticed slight rattle when leaving lights in second gear under load. Why the rattle and why the pressure drop? Have noticed that sound in the past but now seems prominent and consistent at lower rpm under load. From 3rd gear and up sound goes away as soon as I have a bit of boost. What do you guys think.

 

Ya may have a failing or failed fuel injector(s), dumping excess fuel and causing oil dilution, which will cause low oil pressure. Check y our oil level and make sure the level hasn't gone up (term called "making oil"), indicating that diesel fuel is mixing with the oil, and if the oil has an odor of diesel fuel, that should also be an indication.