Cautiously asking why the plumbing is wrong (8,3)

I believe the answer is:

sounding out

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'cautiously asking why the' is the definition.
I can't tell whether this definition defines the answer.

'plumbing is wrong' is the wordplay.
'plumbing' becomes 'sounding' (both can mean measuring a body of water's depth).
'is wrong' becomes 'out' (eg a calculation might be 'out').
'sounding'+'out'='SOUNDING OUT'

Can you help me to learn more?

(Other definitions for sounding out that I've seen before include "Interrogator might be" , "conveying wrong impression to listener?" .)

I've seen this clue in The Globe and Mail.

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