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Recently, something started rattling inside of my less than a year old hardtail mountain bike frame. I was unable to figure out what, and could not fix it. Is this cause for concern? The frame is made of aluminium.

The rattling is audible when riding over rough stuff or cobble stones, when tapping the side of the frame, or when shaking the frame hard with the fork off. There is nothing sliding around in the tube, only rattling.

This frame does not support internal cable routing (other than the dropper), so it is not that. It sounds like it is coming from around the middle of the down tube, or possible top tube. I took out the seatpost to try to take a look, but the seatpost does not communicate with the top tube (other than through a tiny drilled hole). I took off the fork, where I can see an opening into the tube, but there is nothing visible there.

I don't think I can do anything about this rattle, thus my main question: do I need to be worried about it, could it indicate a serious problem? Or should I just try to forget about it and live with it.

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  • Could be part of a bottle cage rivnut which broke off. Or they lost something during manufacturing in the tube.
    – Michael
    Jun 12 at 18:39
  • That the sound doesn't slide when you angle the bike suggests its something tapping in one spot, and not a loose stone in the wrong place.
    – Criggie
    Jun 13 at 2:09
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Try getting some foam sleeve for the dropper cable. Maybe it is just that.

If you want to do an inspection, borrow a car mechanic’s inspection camera. It’s a tiny camera on a flexible stick, complete with lights, that will let you see into the downtube and hopefully find the source of the rattle.

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  • It is not the dropper cable. I removed the seatpost, and it still rattled.
    – Goroga
    Jun 12 at 17:20
  • If you suspect a loose bit in the seat-tube, tape a vacuum cleaner hose to its top-end and turn the frame into any possible direction, since you've already removed the components. The offending bit may be sucked out.
    – Carel
    Jun 12 at 20:43
  • Those tiny cameras are awesome - I got a chinese one for under $20 USD that uses a phone for a display. Its essentially disposable, I'm far less careful with it than an expensive model.
    – Criggie
    Jun 13 at 2:11

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