SLX hubs , 9 months in , something’s making a not very nice noise under load , slow uphill cranking stuff . I haven’t had Shimano hubs for donkey’s years , last time I went through 3 free hubs in the 1st year ! Is there anyway I can tell for certain that’s what it is ? There’s no slippage through the cranks if you get what I mean ? I would have thought if it was the freehub I would have to watch the Crown Jewels on the top tube ! Taking chain off and spinning the cranks produces a horrible droning noise but there is no sideways play when I grab the crank arms ? So BB , Freehub or something else entirely ?

Take the wheel out the bike, remove cassette and feel if the free hub feels rough. The free hub doesn’t really do anything under load, it’s only when freewheeling the bearings are spinning.

Stripping the shimano rear hub is dead easy anyway so you have nothing to lose.

Although if you say there is a horrible noise from the bottom bracket it’s probably that?

id do as above and while its in a state of disassembly, i’d pack the bearings and freehub pawls with grease.