^^ most likely its the contact points of the shoes on the brake backing plate. When you install the new shoes, clean off that plate with the afore mentioned brake clean and get all the dust out of there. Now you'll take a bit of hi temp grease and apply it in the specific locations. Do some searching, you should find an example.
Parking Lot Hero
2008 WRX daily driver
2003 ZX3 23D Autox Car 42 FSP
"Hella Functional - Raced, Not Hard Parked"