Sounds from water in the U-tube?
When a wind instrument is played, moisture from the breath cools and condenses within the instrument. This condensation tends to accumulate at the instrument's lowest point, which for the bassoon is its metal U-tube. This is not an issue while the instrument is positioned vertically during play, but can be a problem when it is placed horizontally after the performance has finished. Moisture can soak into the wood. It is common practice for bassoonists to only rest once they have dismantled the instrument into its component sections, removed the accumulated water, and run the sections through with a swab (cleaning cloth). If cleaning is not performed, the wood in the lower part of the instrument can rot. In winter, water vapor is readily cooled to form liquid, so if playing for an extended period during the cold season a gurgling sound can sometimes be produced.