clanging/dieseling noise under hood!

[Rhett]

All of a sudden, my Explorer has this clanging noise coming from under the hood. It is almost like the "dieseling" sound some people have reported with their trucks. On mine, it sounds more like a bearing has started wearing out in the alternator, or the water pump, or a/c compressor, or fan clutch, or something. It was running fine yesterday, but this morning it was noisy.

Fearing the worst, I checked to see if the truck had oil in it, and it did. Still, I didnt drive the truck anywhere. I thought I had a vac leak around the KKM/MAF so I put everything back to stock there. I also thought the Granatelli MAF might be throwing snake eyes, so I put the stock MAF sensor back on, but it still does it. So now I am stumped. I am taking it to my mechanic tomorrow, but I would like to float the question about what it is here...I think it might be the brushes or bearings in the alternator, or maybe the water pump, or something else up there. It is noisy, like a loud diesel.

 

[Rhett]

My mechanic said it was lifter noise. I've always had a little lifter noise, but never that much. And it went away after a few minutes of running. I'm not sure about using oil additives, as was suggested. The truck sat all weekend so maybe that had something to do with the loud lifter(s) (maybe all the oil drained to the pan). After running, it is quiet as ever. Still, it has never done it before, so it bothers me. I would hope the truck doesn't make a habit of it with only 84,000 miles on it.