I have no opinion on what's making the noise, but I do have a suggestion you should be able to use, to confirm VictorG's diagnosis.
Get yourself a mechanic's stethoscope, like this one:
Lisle 52500 Mechanic's Stethoscope
https://www.amazon.com/Lisle-52500-Mechanics-Stethoscope/dp/B0002SQYSM
I've actually seen some for not much more than $20., that also have a rubber cup attachment, to use as an alternative to the standard "needle." If I buy another one, I'll get one of those, to use in addition to the "needle," but the needle is the one you'll use most.
But for less than $20., you can't go wrong. I've used one for 30 years, and it paid for itself the first time I used it.
WARNING:
Wear safety glasses when using the stethoscope. If you hit a belt/pulley, etc..., the whole thing could get shoved into your face before you even know it. Also, pay attention to any fingers you want to keep--
it's very easy to become distracted, while poking around, listening to a number of very-scary noises, 99.9% of which are normal, but sound crazy-loud, through the stethoscope.
I would just add that (WITHOUT a stethoscope) noises can be very "funny" (read difficult) to track down, as they can "travel" or resonate up and down shafts, through frames, bodywork, etc....
For example, when screwing around in our (frozen) yard, playing with the ALSD/VSC on and off functions, I heard a terrible, rattling sound that truly sounded like it was coming either from the passenger side or the transfer case--but only when the ALSD was on, and there was rear wheel spin. Inside the truck, it literally sounded like a loose transfer case chain, or an almost-engaged front halfshaft--but I was in 2WD and had visually confirmed the front driveshaft was not turning.
With the hood open, I still couldn't tell where it was coming from, but it sounded much less mechanical, and more "thumpy."
Then inspiration hit, and I suspected it was the ABS pump (which is on the D/S), cycling, to work the rear axle's Traction Control function, via the rear brakes. I put the stethoscope on the round, canister-looking part of the ABS pump assy. and, sure enough, there was a distinct "thumping" sound that corresponded with the "brighter," mechanical noises I could hear, without the stethoscope, when the rear wheels were spinning/slipping on the snow/ice.
I've heard ABS pumps cycle before, and none of them ever sound like the mechanical racket we heard inside the cab. But weirder-still, with the stethascope on the pump, it sound far less "mechanical" and more "thumpy," as you might expect.
Happily, the sound corresponded exactly to the timing of the sounds my wife could still hear, from inside the truck, when a rear wheel spun on the ice. IOW, it was perfectly normal--just new, to me. :laugh: And while from inside the truck, it sounded like it was coming from the P/S, or possibly from the center, in fact it was coming from the D/S.
That's why I say noises can be difficult to diagnose.
Again, a stethoscope will pay for itself the first time you use it, and then you can suggest the tech use
his stethoscope, on a cold start-up, in the area VictorG suggested (after you confirm it, first) and that should end the
"It didn't make the noise for me..." part of the process.
Good luck and let us know.
P.S.: Since you're conformable wrenching on dirtbikes, after you get your truck diagnosed, can you try the stethoscope out on one of your (2-stroke) dirtbikes that has (known) piston slap/needs a new top end, and then tell me how to tell the difference between "normal" and "worn out"? (I'm looking to get another bike and all of the old, air cooled era bikes I grew up with sound noisy/"slappy" to me, compared to more modern, water-cooled, two-strokes.)
Thanks!
