I noticed on my ride in to work this morning that my freehub sounded a bit more grumbly than usual when freewheeling, halfway through the ride the noise sounded ‘cyclical’, i.e. there was a rhythm to it the coincided with speed.
I also noticed that when freewheeling at speed down hill the cassette seemed to be momentarily engaging, pulling the chain tight but releasing again when the derailleur hit it’s movement limit.
The hub is a Deore and, I think, cup and cone. Is this likely to be a tighten up or open and regrease type job or more serious?
Usually when the freehub is showing signs of failure, complete failure isn’t far off. I wouldn’t bother trying to fiddle with it. Just whip it off (10mm hex key) and put a new one on. They are fairly cheap if you shop around.