For a fortnight I hear noise from my bike. The sound appears to originate somewhere aft. The bike in question is a disc brake equipped cyclo-cross bike (Merida CX600). Overall weight of bike, rider, and load is a little under 100kg. I shall describe a few observations in order to help to narrow down the cause of the noise.
it comes from aft of the saddle
it has a combination of pling, creak, and scratch sounds
the sound is only audible on smooth surfaces (tarmac)
its frequency of occurrence increases when going faster
it does not sound the same every rotation, it is sometimes longer, shorter, sets in earlier or later
it persists when not pedalling and with no force applied to the cranks
it becomes louder when pedalling
it persist when out of the saddle
shifting my weight forward reduces the noise
shifting my weight aft increases the noise
there is no disc rub (sounds decidedly different) progressively pulling levers does not change sound of the noise
spinning the wheels on a stand does not cause this noise
turning the pedals on a stand does not cause this noise
Beside the noise the bike is new, about 800 km, in very good working order. Some observations on the bike:
The rear brake is difficult to centre, in particular after replacing the wheel. Disc rubbing used to be common until I took a lot of time to set up the brakes.
Something with the front fork, the through axle, hub, or bearings is not right. At side loads it may cause a clicking noise.
I've not checked the chain for elongation in 500 km.
Spoke tension out-of-the-box on this bike surprised me for being rather low. In particular the non-drive side of the rear wheel had and has rather soft spokes. I checked both wheels 500 km ago for centring, they were turning perfectly true.
The bike is ridden for 40% on tarmac, 50% on smooth gravel, 10% on rough terrain (by distance).
My suspicion turns to the spokes. You've said they're a little loose, and one looser than the rest could easily make a noise only when there's weight on the bike, but with no need to pedal. A slight tighten of all the spokes on that wheel might sort it out, while keeping the wheel true. You may also be able to feel/hear a loose spoke by plucking, and confirm by putting something like tissue paper between that spoke and the ones it touches. An unusally tight spoke may also be the cause.
From my experience, I now have three ideas about possible reasons.
You have some dirt on your seatpost. It's strange how much noise it can produce. Try cleaning it really good, use some soap on it etc. May be clean the seatstay too. Small pieces of sand can go there by the seatpost clamp and make all kind of noises.
Check your tires. May be they are under- or overpressured. It may be just the sound of your tire (try different tire).
May be you have some damaged tubes, for examle, in dropouts and tire touches tubes when you seat.