Pal's constant noise (4)
I believe the answer is:
chum
'pal's' is the definition.
(synonyms)
'constant noise' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'noise' becomes 'hum' (hum is a kind of noise).
'c'+'hum'='CHUM'
(Other definitions for chum that I've seen before include "Informally, a close friend" , "Good friend" , "Pal, mate" , "Salmon" , "Pal, buddy" .)