jengel

High frequency vibration comes on when coasting. Goes away when I start pedaling. Feels like a vibrating bed machine in my saddle.
Older Cannondale Carbon Synapse. Can't find any frame cracks. Any ideas?

FBinNY 

Since it's only when coasting, it's some kind of freewheel noise. Depending on whether it was always this way, came on slowly or suddenly, or other info, it could be inherent to your freehub (some sound like buzzsaws), lack or lube, or rust ot other issues within the freehub, or much simpler something would up against the hub behind the cassette causing noise or drag when they don't turn together.

If this bike isn't new and the noise is, look behind the cassette, and/or remove the cassette and clean there before taking th ehub apart looking for an internal problem. Once the cassette is off you can test off the bike spinning the wheel while holding the freehub body.

HillRider

Does the bike have a "dork disk" (the plastic or metal spoke protector behind the largest cog of the freewheel or cassette). if so and if it has slipped or cracked, that could be your source.

gyozadude

Step on the BB first on one side and then from the other side and check for flex. When you pedal, there is chain tension that flexes the rear triangle toward the drive side and compresses the right chainstay to the BB. When you coast, you might not be compressing that member and it can set up a vibration. It's curious that you feel it coming up through the saddle. That would concerned me. Usually a freewheel vibration wouldn't be that bad.

dddd

If you're using a high-end Mavic wheelset, the freehub bodies on these require regular lubrication, every couple of thousand miles is a good time to clean up and re-lube.

I do this at home, but you should first know what you're doing.
Loud noises while coasting can also result from a neglected freehub and bushing.

cny-bikeman

Vibration can mean a lot of things. Is it a buzzing (very frequent, small or imperceptible movement) or more like a pulsing (slower, larger amplitude)? Pedaling may just mask the vibration or can short-circuit whatever harmonic vibration is happening.

Continuity

+1 - Indeed. I found this to be the case when servicing my Aksiums. (although they're not exactly 'high end' )

Do you bother with the recommendation to always replenish the threadlock on the locknuts and and to always use a new 'compressible adjuster washer' like Mavic recommend?

I was happy to find out that somone one Ebay in the US offers fairly reasonably-priced replacements for those weird nylon (or Delrin, whatever) bushings that Mavic use, as well as an 'oversize' version that's a couple of thou of an inch larger so as to fit snugly on hubs that are a bit worn.

dddd

I've bought and used the replacement bushings, but even the std-sized ones often need a little relieving to turn smoothly.

For freehub servicing, the compressible washer isn't disturbed. It's under the bearing adjuster ring.

And, for routine axle pulls, I don't use the LocTite on my own axle. But it can help keep things tight over the unknown conditions and possibly longer interval of someone else's hub. I have found them loose on occasion, and have seen many such hubs with bearing-loosening issues and which thus required frequent adjustment. A very low strength of LocTite might help with this on the adjuster ring.

jengel

Thanks, all, for the help. It indeed was the plastic bushing in the Mavic wheel. I'll order a new piece and be on my way.

Continuity

Are you ordering it from the US guy on Ebay who sells them?

And did you try cleaning and lubing the one that's on there now? They
do tend to make a terrible racket when they need TLC.

jengel

Yes, I'm ordering from ebay. And the lube is ok. Not making noise. I'll, of course, clean and lube before I put it all back together. There is significant play between the plastic bushing and axle.