Out in the rain yesterday on my faithful Audax bike and I started to notice a rumble from the drivetrain that became fairly pronounced and then faded though not altogether - very much like chordal effect and mainly in the smallest 3 or 4 rear cogs/large front ring. Almost felt like something had gotten lodged in jockey wheels and I put it down to muck being kicked up from the dirty road due to the wet (or all the lube being instantaneously washed off!). Too wet to see anything TBH.

Age of parts (oldest first and ignoring the from 105 changer which isn't touching/dragging)
Chain - shimano - fitted late October '16 (regularly cleaned and wet-lubed)
Deore LX M591 rear mech late October
chainset - triple 24-36-46 Stronglight Zircal rings - Jan '17
BB - Stronglight JP400 - Jan '17
Cassette - XT M770 9spd 11-34 March '17
Hub - Shimano FH-5800 105 March '17

(Minor chordal effect noticeable in old wheel (XT hub with similar cassette but 12-28) but really only in smallest ring and not anywhere near the same level)

I've degreased and lubed the chain and regreased the lower jockey wheel. The hub and BB are smoother than butter (I even removed cassette and inspected hub). I've done 700 miles on that chain but I'm skepitical it would be the chain given the way in which the problem came on so I'm loathed to break it and replace it (especially as I hate these these new Shimano 'break-the-pin' chains) though I do have one ready.
Any thoughts?

Just one: you should test ride the bike since you lubed it.

Sorry, I forgot to mention that I popped it into the turbo trainer to check it

Pedals?

Two new M520's last month (also buttery smooth )

I'd guess you just need to pimp more brands.

Even if it's shifting OK, might just check the RD alignment? Maybe something's getting tweaked in specific gear combos?

Something else that I failed to mention - I did a quick alignment check with a spare 10x1 rear spindle in the derailleur mount and a steel rule and it looks pretty spot on (though not as good as using a Park tool). The onset and then fade of the problem (though still not gone as mentioned) would I doubt be explained by the alignment (and the older XT hub and 12-28 is fine).
I'm thinking it might be worth changing to another chain, maybe a KMC or a Wipperman (thought I can't seem to find the 908) - still doesn't really explain the phenomenum but maybe worth getting a new one (or two) anyway

Well it turned out to be the chain. I mail-ordered a Wipperman Connex but picked up a SRAM locally and it's right as rain even in the 11 there's virtually no noticeable vibration - I guess 700 miles over winter is enough wear to create an issue with a new cassette. Oh well, you live and learn (and thanks for all the feedback).