Vrs octavia squeaking noise from engine

I get a squeaking noise (chik chik...) from the engine when I run it at 1200 to 1800 rpm and it is pronounced with the ac on. The noise stops after about 2000 RPM.

Took it to the dealership n they changed the alternator belt, lubricated the Idler pulley of the alternator belt as well as idler for the timing belt. The noise has'nt stopped. My car's done about 12000 Kms and is 1 year old.

Now they are saying that it may be from the alternator or th ac compressor (bearings). Can anyone tell me what it actually is?? Ive been to th dealership thrice n they seem to be lost.