paps sanik,

i think it's natural na bumaba ang idle because the ps pump is getting power from the engine. tuwing kumakabig kasi nangangailangan ng power yung ps oil pump thus the low idle...kapag straight runs hindi kumakain ng power (relatively) yung ps pump. as long as walang maingay na buzzing sound it's normal. mapagkukumpara mo naman siguro by experience yung normal buzzing sound to the abnormal sound.