I would try to push and pull a cable or a hose from the back in the chainstay in various directions to maybe catch a debris stuck there.
Already done that. Compressed air, vaccum with a slim hose through the BB, long bottle brush. No effect.
I would try flushing the frame out with water, if you’re able to. Like, a lot of strongly flowing water. Shaken up inside the frame if possible.
I would be hesitant to do this. Not sure what the long-term effect of any trapped moisture would be, especially given that the BB shell is a bonded insert.
Carbonda's "solution" was hilarious though. They advised to "
drill a small hole inside of seatstay and fill with foam spray". Yeah, no!
The frame is otehrwise lovely and the other one is faultless. Just going to have it built up.