noise at 17mph
 

My buddy has a mountain bike with disc brakes. Dunno if it's the brakes...but the drive train is basically silent till he hits 17mph and then there is a metalic chirp from the rear end. Pawls in the freewheel? Brakes? When he goes faster or slower it's gone. Very odd. any guesses? He doesn't have to be peddling for it to happen. It not completely constant at 17 but it's fairly active at that speed and only that speed. It's easily heard even by a second cyclist riding nearby.

Can you be any more specific about the sound? It's extremely difficult to diagnose any sound in print, so details about how often, how loud, comparison so some other sound that's similar (bird chirping, door squeeking, etc. would help. I assume is happens irrespective of gearing, temperature, road type?

that's the signal that something is vibrating on it's resonance frequency, which happen to be at 17mph (27.2km/h) Assuming 675mm diameter => 2.12m/revolution=> 3.56Hz (3.56 revolutions of the wheel/second)
At this low frequency only a spring can achieve resonance. (those 3.5hz are multiplied by the number of ratchets in the one-way mechanism. so it gives 30-50Hz which is almost for sure a spring)

So if the sound is present only when coasting at 17mph then most plausible are the springs of the pawls.
If the sound is present whether coasting or not, then it could be the balls in the bearing of the hub (if it's loose ball configuration, the balls are actually clicking by bumping each other at regulate intervals, when the frequency match, that clicking is amplified)
Or maybe a spoke is loose, or even the classic rattling of the valves nut to fix them on the rim.

Quick tests: check the valve cap, and valve nut, check for loose spokes (it should be extremely loose to ding when riding)
If all is good, repack with thick grease the hub bearing.
If nothing changes, then flush the freewheel or freehub with mineral spirits/diesel fuel to clean it, then oil it with some oil (like 75W90 manual transmission oil) by squirting with a syringe while revolving the freewheel or freehub so it wil suck the oil by capillary action.
The disk brakes should not be a problem since it either wobbles and generate friction at any time (and causes drag, but clicking noises are most improbable, maybe only intermittent "friction sounds" not clicks), or if they don't wobble and they are not in contact with pads then nothing can be heard.

Also look at ANY nut or screw on the bike (for fenders, racks, lights, dust covers of cranks, brake bolts, brake disc bolts, etc) it may be loose and at certain frequency the washer/bolt/nut is vibrating.