Water cooler sound (4)
'water cooler sound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for glug that I've seen before include "Drink" , "Hollow gurgling sound" , "Noise of emptying bottle" .)
(Other definitions for gurgle that I've seen before include "Baby's noise" , "Hollow bubbling sound" , "Rodent snare" , "Lugger (anag)" , "drain sound" .)