Possible Clues:

• Sounds as if one knows where there's such a hole
• One knows the sound of the whole of this on the face of it
• Opening number's warble cut short
• The boy's up on time! Right, one student can have a breather
• Air intake spot
• One knows, by the sound of it, where to find a 33 down of this
• One knows where to find the whole of this, by the sound of it
• Air intake cavity
• Zero silt spread around river -- that allows flow under bridge
• Nose aperture