Ethernet Wiring

FWIW:

NEC (National Electrical Code) 300.22(B) does not allow an Ethernet cable to be installed in HVAC supply air fabricated duct work for the use as a raceway.

Even the exception doesn’t allow its’ use as a raceway.  

300.22(B) Ducts Specifically Fabricated for Environmental Air.

Exception: Wiring methods and cabling systems, listed for use in other spaces used for environmental air (plenums), shall be permitted to be installed in ducts specifically fabricated for environmental air-handling purposes under the following conditions:

(1) The wiring methods or cabling systems shall be permitted only if necessary to connect to equipment or devices associated with the direct action upon or sensing of the contained air, and

(2) The total length of such wiring methods or cabling systems shall not exceed 1.2 m (4 ft).

https://www.electricallicenserenewal.com/Electrical-Continuing-Education-Courses/NEC-Content.php?sectionID=276

NEC does allow Ethernet cables to be installed in air return plenums provided the cabling is UL Listed plenum rated. (Or, NRTL Listed meeting UL)

FWIW, If the OP does install the Ethernet cable in the supply duct work he should install an insulated  grommet or bushing in the cut entry hole in the metal ductwork to protect the outer jacket, and internal insulated conductors from being cut, damaged, by the supply air movement through the duct chaffing the cable at the sharp cut entry hole of the metal duct work. 

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sns

The physical distance between the modem/router and streamer is of little or no consequence ...

My audio system works best with the modem and router located away from the system and connected to the streamer via ethernet. The streamer sits in my audio system rack.

... If you’re worried about internally generated noise migrating via LAN cables ...

No, I’m not worried about that at all.

... modems and routers with wifi disabled, sitting in close proximity to any or all other streaming components will not contaminate those components in any way.

You may be correct. But of course my system includes other components, too.

@cleeds If any of those other components require wifi I'd rather not have them in my audio system. I run my home theater and every single wifi device in house off 'dirty' side of local network. 

Clocking/timing/synchronization also important for optimal networking, taking output directly from router into streamer not good idea, this where LAN filters, switches, reclocking devices should be placed. 

And the above brings me back to the idea of implementing an audiophile managed switch. One can eliminate noisy router,  filtering/isolation and reclocking all taken care of in one device, no need for all these add on devices/complications.

The single most important thing to remain mindful of,  baseline noise floor is set by the noisiest component in streaming chain, for most that will be the router. Implementing all the finest network components downstream of noisy router is rather futile as router remains the bottleneck. 

Also, don't forget a wifi enabled router not only contaminates nearby components with EMI-RFI but is greatly contaminating itself, max isolation of other components/parts within router is of very little concern to these manufacturers. 

sns

If any of those other components require wifi I'd rather not have them in my audio system ...

My audio system relies on wired ethernet. I don't use wifi for audio other than to control the streamer.

Clocking/timing/synchronization also important for optimal networking ...

"We hold these truths to be self-evident."

... don't forget a wifi enabled router not only contaminates nearby components ...

I don't use a wifi router.

@cleeds All remarks not directed specifically to you. 

Just curious, you state you have modem and router, so how do you provide wifi? While you may not be using wifi with any audio streaming components it still affects your network. I also use only LAN connections with audio streaming components, and have been for well over a decade, disabling wifi and providing 'clean' side home network was beneficial.