If the noise occurs when you're not braking, the backing plate may have
been bent during servicing and is now rubbing the rotor.
If it occurs when braking, it's possible your mechanic is correct. I
haven't heard of any issues with chemicals causing this, but I suppose
it's possible that it could be due to some interaction with the rotor.
Some rotors come with a painted surface and make a similar noise until the
paint wears off. It's also possible it's a noise characteristic of the
brake pad material.