It uses a fusion in-between of:
-Idle sound of the Caprice Sound 1995 Chevy Caprice Sounds Mod + LAPD Siren by GTAEU-MODS
-Lower RPMs from  Mercedes Benz SLS AMG Engine Sound MOD V2 by rkmario
-Higher RPMs from the  Mercedes-Benz AMG Sound Mod by DeathTroll