engine clatter - from where?

[Ian7]

Sorry, this will be long post by necessity.
New-to-me 2004 3500/Allison (LB7?) with 115k miles, bought sight-unseen (don't go there, it's water under the bridge).
Starts easily, runs well, pulls the trailer well. Good oil pressure, no apparent leaks or other external anomalies.
But it makes a much more apparent, odd "clattering" sound more than other diesel duallies I've been around over the years. At idle, it makes an intermittent (as in does it most of the time) distinctly staccato clatter that I can only describe as being like someone sending morse code, but a 'deeper' sound that to me sounds like valve clearance problems. But if that, why inconsistent and erratic?
Also, at medium steady-state throttle (65mph) what sounds like the same sound is there, but its absolutely steady, (not the morse code erratic) and rises in frequency as the engine rpm climbs. Oddly though, a quick throttle lift silences it completely (every time I try this it always goes away), but the moment I add any throttle, even as little as required to halt the deceleration, the clatter returns (always).
One "expert" says I need new injectors. Not sure I agree, as I just returned from a 2100 mile, 65mph highway trip and averaged 19.3mpg (which to this layman says the injectors are fine). Thoughts?
Another says "valve adjustments", but again, why the erratic, only most of the time noise at idle?
A third says rod knock, and "junk the truck"....
So, I have $12,000 in this truck. All of the above is hugely expensive in relative terms, so looking for advice on what it might be, and equally important, what NOT to waste my money on. Thanks in advance, you guys are great.
(I have iPhone audio of all this which may or may not be useful if someone wants it emailed to them to listen to).

[DmaxMaverick]

This sounds all too familiar. You have one shot (maybe two) at good news. Email the sound clip to me and I'll see if we can make that a possibility. Email is in my sig. Otherwise, I'll withhold speculation at this time.

[Roy W]

It sounds to me like an enrichment circuit problem. The way it was explained to me years ago was kind of like an accelerator pump circuit on a gasser.

[DmaxMaverick]

I got your sound clips.

This does not sound fuel related. It is mechanical, which isn't good, but may not be as bad as it sounds. You'll need to repeat your sound tests, first as before, making sure the noise is present. Then immediately again with the accessory belt removed (don't worry about the instrument panel complaints, and CA emission engines will throw some codes). Good news would be if you can't repeat the noise with the belt removed, meaning you'll have to identify which accessory is the cause. Spend some time with a stethoscope and pinpoint it. Not so good news if the noise is present with the belt removed, meaning it is internal to the engine. First, pull the glow plugs and make sure they are all intact (no missing pieces). At this point, also check the flex plate and torque converter. Either of these broken can make similar noises. Good luck, and keep us posted.

[Ian7]

I got your sound clips.

This does not sound fuel related. It is mechanical, which isn't good, but may not be as bad as it sounds. You'll need to repeat your sound tests, first as before, making sure the noise is present. Then immediately again with the accessory belt removed (don't worry about the instrument panel complaints, and CA emission engines will throw some codes). Good news would be if you can't repeat the noise with the belt removed, meaning you'll have to identify which accessory is the cause. Spend some time with a stethoscope and pinpoint it. Not so good news if the noise is present with the belt removed, meaning it is internal to the engine. First, pull the glow plugs and make sure they are all intact (no missing pieces). At this point, also check the flex plate and torque converter. Either of these broken can make similar noises. Good luck, and keep us posted.

many thx for the input, will let you know.