Only on descents - I'm stumped (and kinda scared)

[lph]

Just thinking out loud here:

if it "feels" like your brakes hanging up, then something is giving friction. The vibrating could be the handling of the bike itself, some bikes do that at speed I've heard, but with screeching and shaking and a feeling of resistance somewhere it's not just "chatter".

It sounds maybe like a broken or cracked ball bearing, that only sometimes gets caught in a position where it starts complaining. Maybe under lower speeds and less pressure it never gets stuck in that position but keeps rotating. I don't know what a freewheel looks like inside, but I think there are ball bearings there too, and it could be a hub of course.

I've opened up and overhauled the ball bearings on my mtb hubs many times but on my road bike I just bought new machined bearings (wrong term, maybe, sealed bearings?) and my lbs thumped out the old ones and put them in. Done in a few minutes.

Good luck figuring it out.

PS. Hey, I just read a good tip on the RoadBikeRider Newsletter - swap the part you're suspicious of and see if the noise goes away. Try swapping out one of the wheels and free-wheeling down the same hill maybe?