What's this Knocking from rear hub when coasting at high speed

Question

I recently started to notice a rhythmic clicking sound coming from the rear of the bike, when coasting, several weeks ago. I ride an average of about 10 miles per day. This sound was intermittent, and occurred only when coasting, in addition to the sound of the freehub. Now, I noticed that at high speed on descents (~30mph) a similar sound begins to occur when coasting. It's louder, presumably because there's more force at high speed. It sounds to me like a sticky pawl? But I'm not sure how to diagnose that myself.

I asked at two local shops, who said the wheel is fairly generic and it's probably not cost effective to service the hub, versus replacing the wheel down the line when it actually fails. I'm more concerned about the mode of failure, though - is my hub at all likely to seize up at high speed and send me skidding? Or something like that? Should I insist on servicing it, before it gets any worse?

The bike is a Surly Straggler Apex 700c from 2021, which has SRAM Apex 1x11 drivetrain, the hub is all black and does not have any apparent brand mark. The bike has about 2.5k miles from the past year, riding through four seasons New England commute, so it's had lots of wet and salt and not that frequent cleaning.

Answer 1

Odd - when coasting the cassette and chain etc are all stationary relative to the frame. That implies the noise is coming from the freehub in the middle of your cassette, and only while the pawls are disengaged and the wheel is turning faster than the freehub's body, so it should click evenly and regularly.

If you take the rear wheel out of the bike, can you cause the noise by spinning the cassette?
If yes, take the cassette off and do it again, if you can still cause the noise, remove the freewheel from the hub, see if the noise comes from the freehub.

It may be possible to service the freehub, or it may be easier to simply replace it with a new identical one. You won't know till it is apart.

Answer 2

Two things to check for are play in the bearings, and a bent axle.

I don't fully understand why, but the clunking I've had from those (especially when a bent axle meant the cones loosened repeatedly) was more noticeable when coasting than when pedalling, at least at first.

Also check that the freehub body is tight on the hub (some take unobtainable tools to tighten, e.g. JoyTech), otherwise it can wobble as the wheel rotates. When you do so check that the cassette itself is secure; while that alone shouldn't be able to cause the noise, it should still be secure.

This year at least, the hub is apparently Novatec, which I've never handled.

I've had freehub failures far from home, and they make me unhappy, so if you're doing occasional remote rides where the consequences of getting stuck are bad, you should probably get it dealt with now. If it's a commute with a plan B, you're much more likely to be OK so long as nothing keeps loosening.