Howdy everyone, I’d like to ask a problem that’s been going on with my truck that happens probably once a year and very very randomly, last year 2024 when I was idling for almost 20 minutes, just idling, I heard a loud THUNK, I tried looking for where that sound came from and nothing, now this year 2025, a few minutes before I posted this it happened again THUNK, only when idling. My truck is a ‘87 5.0 EFI 2wd

 

There's no way anyone can give you an accurate answer to a noise. It's likely to be coming from something that the serpentine belt is driving. You could disconnect the belt and check each pulley for smooth, noisefree rotation.

Another option is don't idle for 20 minutes, it's not good for an engine anyway.