Dura Ace 7800 drivetrain.
When I'm out on the road with the wind shrieking in my ears, traffic, bumps & other distractions, I cannot sense it, but when I'm on my rollers, I feel (but do not hear) a very faint grinding feeling from my BB or possibly chain.

It only occurs on the small chainring and on the smaller rear cogs. (The closest I EVER come to cross-chaining is 39-14 (two small cogs are off-limits when on small chainring).) I can minimize it by focusing on perfect, fluid form and high cadence, but it's always there.

Until I shift onto the large chainring, then it disappears completely! Even when on my third-largest cog. (i.e. the closest I get to cross-chaining the other way.)

Chain wear doesn't appear to matter. It has occurred on my last three chains, and with different cassettes. I currently use an SRM crankset, but it also occurred with my OEM Dura Ace crank.

The BB doesn't appear to be the culprit--if I remove the pedals, the cranks spin completely freely (we're talking a dozen or more spins after I stop manually spinning it) and there is no noise or grit whatsoever.

Any ideas?

I would check for a slight chain rub on the front derailleur. You should have at least 1/32" (.75 mm) clearance when there is no pedalling load. Sometimes the FD is in need of a slight rotational tweak about the seat tube axis for optimal clearance in as many cogs as possible.

Thanks very much for the suggestion Steve, but I checked that and it's definitely not even close to touching, even when crossed to 39-14 (third smallest cog.)

It doesn't make any audible sound, it's just a faint rough feeling I can sense through my shoes (and perhaps also in the handlebars.)

I've also checked the chain slack, and none of the pulleys are rubbing anywhere, even when crossed to 39-12 (which I never do.)

I've noticed the same or a similar sensation on the smallest cog in back. This is true of several different bikes and I have decided it must be related to the cog having so few teeth. I'm not sure how to explain it, but I feel pretty certain that has something to do with it. At any rate, after several years I still notice it and there have been no problems developing after thousands of miles.