Am I damaging my induction cooker by putting piece of metal under aluminium pan?
No. 

Your induction stove don't care what it's heating as long as the resonant frequency and impedance stay within specification and unless the manufacturer really screwed up, it's protected against it. Case in point, does it detect when a pot is removed and start beeping? That's the protection kicking in. 

Hissing noise is unfortunately normal for induction stoves.