I guess I am 'that guy' today, this is making me crazy. This is about the drivetrain of a MTB triple.

Under load, I get a loud click at one point in the crank revolution. Exact same place in the rotation every time. Only under load - I cannot reproduce the problem on the stand, I need to be on the bike and spinning up. I have replaced the chain, inspected the bb cartridge, looked for any burrs on the jockey wheels or chainrings, looked for any interference with the FD, reseated the cranks. These efforts have yielded no difference. So far, this has cause zero difference in feeling or handling of the bike, just a noise.

I'm stumped. The spindle is a sealed cartridge, could it be the bb cartridge has gone bad? It is completely smooth turning to the touch.
What would you try next?

tighten chainring bolts

Pedals?

bottom bracket...

Try swapping your pedals with a known-quiet pair, preferably platform type to eliminate cleat and shoe noise.
Clean and lube the threads and the mating surfaces and torque correctly.

Do the same clean/lube(threads only)/torque the chainring bolts.

Ensure that a cable end is not "ticking" against the crank or your shoe.

Try pedaling out of the saddle to rule out saddle/rail/seatpost noise.

Pull the BB and clean/lube threads/reinstall with correct torque; 36-50 lbf-ft for Shimano.

+1 And after all of this and it still does it ,yes replace the seal bottom bucket .

Thanks to all for the good ideas. I changed pedals and if a short ride tonight is a true indicator -- problem solved. Rotating the offending pedal spindle with my fingers, it seems like the bearing is shot, feels like metal rubbing on metal. Wild that the symptom is one loud click, but I'll take it as a win. Better problem solved in the garbage than at some LBS 50 miles from home, and I can proudly say I out road my pedals.. Long live the BF.

My last mysterious pedaling click ended up being a seat post that needed to be regreased.

I'm glad you apparently have solved the problem, but I'm going to add an observation/suggestion for others with similar problems, or just for future issues.

It's important when approaching a mechanical problem to use observation and logic. A noise that only occurs in time with crank revolutions and only under pressure has to be connected to something that is stressed at that time. The chain, cassette, and hub/wheel rotate at a different rate than the cranks - all but the chain rotating proportionally different depending upon gear, and the chain in proportion to the links of the chain vs. chainwheel teeth engaged. The jockey wheels are under the same (minimal) pressure at all times. You were correct to check the chainwheel and cranks, and the focus above by bikeman on chainwheel bolts, pedals and BB mounting was also appropriate. More rarely parts outside of the crankset/BB can be involved, as saddle, dropouts and handlebar can also be stressed in time with crank revolution.

If you think about the engaged parts of a bearing which roll past each other (cone/cup/balls) there is also not a good argument for them causing a clicking sound at the same point every revolution - at least it's relatively rare. The mounting of the BB in the frame is more often the problem than the bearing itself. Although the pedal seems to be the culprit in this case, that's not necessarily the case. It could merely be that in the process of changing pedals you either tightened the new ones better or some foreign matter between pedal and crank arm face was removed. The proper procedure is to first remove the pedals, clean the interface (I recommend 000 steel wool) and firmly remount. Different pedals is done as the next step. Also, when pedals make noise it can be a problem in the pedal frame - cage not secured properly to the center, for example - not necessarily the bearing. Hope this helps others and the OP in the future.

I think Sherlock would be surprised by the number of variables. Yes, it is only logical to assume the problem exists in a component sharing the same orbit as the crank, but..then add in the load variable, then directional force - (it doesn't present going backwards), then the center of gravity change through the riders pedal stroke, and on and on. Before you are done you have involved pretty much every part of the bicycle as possible causes, including the seatpost and headset.

Anyway, the problem is solved. A quiet bike is a joy indeed.

Yes, there are several possibilities, but it's only sensible to first eliminate the ones that make no sense, then to focus on the most likely (those right at the crank/bb area). The entire rest of the possibilities are probably 1% of the total.