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Shimano Deore xt, 10sp. The noise happens only when I change gears for the short period of time when the chain is moving from one sprocket to another. While pedaling on one gear it works well and silent. The chain is not new, but is not worn out, no skipping or jumping etc.

What can be a reason for this?

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That whistle noise most likely is from the jockey wheels, or at least that's always been my experience. It seems to be caused by the side load placed on the jockey wheel from pushing the chain. Once the shift is complete, and the side load is gone, the noise disappears.

The quick & easy fix is to first clean the outside of the jockey wheels real well. Then lay the bike on it's side and carefully drip some oil (3 to 4 drops) into the gap between the jockey wheel and the dust cap. Surface tension will pull the oil into the jockey wheel. Then flip the bike over and lube both jockey wheels from this side. The goal is to get enough oil into the jockey wheel that it lubricates the internal bushing. Excess oil will weep out the bottom of the jockey wheel so give it a quick wipe after going for a few miles.

The thorough option is to remove ONE of the jockey wheels and completely clean the wheel, bushing and dust caps. I like to put a bit of grease inside where the wheel and bushing rub. A thick oil will also work. Reassemble and install the jockey wheel. Repeat this process for the second jockey wheel. By doing one jockey wheel at a time it's impossible to reinstall the wheels in the wrong location.

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    I've done exactly the same, then went for a ride and there were pleasant 25km of silence! But while cleaning the jockey wheel I've noticed it's quite worn, so maybe it's worth replacing.
    – k102
    Feb 27 '20 at 10:37
  • Agreed, if it keeps coming back that's likely the only solution but I've not had this happen yet.
    – NoCo Rider
    Mar 1 '20 at 6:00
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I've come across this before, but never on my own bike. The real cause was never identified, but I believe stripping, cleaning, and oiling the cage and jockey wheels was the fix. That leads me to suspect that a little sideways pressure during the shift caused something, like a bit of grit, to rub.

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  • Thanks! Should I consider replacing jockey wheels, or oiling would be fine?
    – k102
    Feb 25 '20 at 8:44
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    Could be some kind of resonance. Damping the shifter cable that runs under the chainstay by threading it through a length of insulation from an electric wire might alter the resonance frequency and stop the noise.
    – Carel
    Feb 25 '20 at 8:59
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    Definitely clean and oil first; that's free apart from your time
    – Chris H
    Feb 25 '20 at 10:50
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    @ChrisH made this - seems that it helped, but not sure for how long. will go on a ride and then test
    – k102
    Feb 26 '20 at 16:11

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