when I cycle on the road steed and freewheel there is next to no noise.

do it on the mountain steed and its like I am being pursued by a swarm of hornets.

why?

you may be riding faster than the speed of sound on road.

you may be riding faster than the steed of sound on road.

bigger volume tyre lets the free hub resonate more?

Obviously, there is no control here, it would only be a really fair test if the same hubs were used on both wheel builds.

shimano hubs on the roadie, hope on the mtb by any chance??

kings on the mtb – always been noisey. but never had silent hubs on a mountain steed!

My guess is that the mtb’s lower gearing can exert greater torque on the hub so the ratchet has to be stronger with bigger pawls which means more noise.

Your guess is wrong.

OP what hub on road?

I reckon that road hubs are often quieter because the speed of engagement is less crucial and therefore they need less pawl/springs inside, which also reduces weight. And it obviously depends entirely on the particular hub design!

Freehub internals are the same on road vs mtb for most if not all
manufacturers

Design and speed you’re travelling at. Old campags are loud and nice

Freehub internals are the same on road vs mtb for most if not all
manufacturers

Aye, true, but the quicker pickup and light weight are very mtb/road orientated. IIRC the hope, mavic and shimano are identical pickup, so I agree. When I design my own super road and mountain specific hubs I’ll make them specific 🙂