Dampen vibration noise in duct initially, condition unknown (8)

'dampen vibration' is the definition.
'humidify' can be an answer for 'dampen' (humidifying is a kind of dampening). I'm unsure of the 'vibration' bit.

'noise in duct initially condition unknown' is the wordplay.
'noise' becomes 'hum' (hum is a kind of noise).
'initially' says to take the initial letters.
'condition' becomes 'if' (as in 'a big if').
'unknown' becomes 'y' (algebra).
The initial letters of 'in duct' is 'id'.
'hum'+'id'+'if'+'y'='HUMIDIFY'

Can you help me to learn more?