squeaking on braking

[Anonymous]

hi guys
my  2 has a loud squeaking noise coming from the rear wheel on braking  a kind of tight spring noise    
i'had this problem for 12 months now and i'm at the end of my tether   s:( s:(  , at last years service i asked them to take a look at my brakes they said my caliper was sticking and they had greased it , it did the trick for a couple of months then it came back same noise but louder, at this year service i  asked them to change the offending caliper  but they say my caliper isn't sticking and they don't know what it is  s:? :? s:?  .

any ideas on what it could be

thanks in advance