So replaced my chain this morning as the last one had snapped in two places. Like for like chain (kmc x9). Now when I pedal hard in the middle cog (any gear on the back) there is a distinct clicking sound from the right hand pedal/crank arm towards the down stroke.

Pedalling hard on the left hand side does not cause this, and even stranger pedalling hard in granny or big ring under similar pressure does not cause and clicking.

A bit befuddled. Can chains click/pop?

Worn middle ring I would most likely think. Not seating very well with new chain.

Chainsuck

Pedal bearing or BB bearing? Loose crank arm?

What sort of BB are you using? If it's sq taper or octalink/isis BB then remove the right arm grease and re tighten.

Dont grease a square taper- just the bolt theads.

cable end getting hit by crank arm?

What bike. If it's an orange five its prob a bit of grit in the free hub or under cassette. Makes a distinctive and annoying click that you may not even here on any other bike due to the swinging arm design. It may well be that on any other bike too if you have tried all of the obvious places.

Like johnb, I'd have thought pedal or BB bearing

Tis a hardtail so shouldn't be suspension related

I thought at first it was BB/pedal but if it doesn't make the sound in big or granny ring I kind of ruled that out.

Only other thing appears to be worn middle ring, but none of the teeth look shark fined…

Should have just kept the old chain and worn it all out it would seem….

Bent chainring? I had something similar on an old race face ride xc years ago. What crankset you using? Failing that I would go for crank arm or pedal. Strip it down, give a good clean and re grease and tighten up.

Front mech cable-stub catching on the LH crank arm?

(My theory: It's only in the middle ring as the cable pokes out most in this gear. Plus it only catches the LH crank on the back of it's rotation hence doesn't do it when pushing hard on the LH crank as the crank is at the front when pushing down)

chainset is an xtr 960 so hope it's not a bent ring 🙁

I take a look at the mech cables on tomorrows ride. Couldn't replicate it on the bike stand tho.

My mate had a clicking on his bike while we were out today, turned out to be a loose chainring bolt, worth checking 😕