It could be just the particular type of radio kit used by he airline. Historically, many a/c types had distinctive 'transmission sounds'. The VC10s were easy to spot, so were Vanguards. The BAC1-11 500 series had a distinctive 'squeak' at the end of each Tx.
Pan Am had a distinctive sound too, irrespective of a/c type.
I wonder if the crews are using microphones/speakers instead of headsets, and it's ambient noise?