I'm looking at picking up a Super Duty with a 5.4 in it from a local salvage yard. It's got a clean title, and doesn't need much work, but it's got an engine noise that has me a bit concerned. At idle, it has an occasional knock, as well as a tapping sound. The tapping sound could easily be the injectors and what appears to be a little exhaust leak. Never had a truck that didn't make some kind of noise like that. The more pronounced knock concerns me, though. If I hold the engine at 1,000 RPM, it goes away, but if I let it idle down from that, it comes back, fairly quietly but distinctly, and not consistent at all. If I rev it past 1,000 RPM, it gets louder, but still doesn't have the rhythm I'd expect from rod knock. You can hear it clearly from inside the cab, under the truck, or with the hood open looking in from above. Can't quite tell exactly where it's coming from, but it's definitely engine noise. I've heard noises like that before, but only in engines that were on their way out, but it's usually a consistent noise.

I've seen posts about cam phasers and timing chain slap, but usually it seems that those noises have a fixed range of RPM you'd hear them at, usually low end. What confuses me is that it apparently goes away if I hold the needle on 1k. I can get a used engine for a decent price, but I've torn a few of these trucks apart for scrap now, and don't look forward to having to do an engine swap.

We've had some older trucks and tractors that made all manner of nasty noises and ran for years, but I don't know anything about the modular engines. My newest engine block up til now has been a Windsor from the early '90's. Any thoughts?