Noisy airconditioning

Rubber feet are certainly worth a try, but I guess the lady downstairs is pretty sensitive to the sound and she'll still notice it..maybe she'll get used to it.  Perhaps  a large G&T before she retires would be an idea!

try mounting it on polystyrene - that usually absorbs some noise and vibration. 3-4 cm

If you go down the 'rubber feet' road, try refixing the wall bracket with rubber feet between it and the wall - this will minimise ant transmitted vibrations. However,  I suspect the problem is more 'fan noise' from the machine, and there is not much you can do about this.  You could look at fitting a horizontal panel below the unit [a shelf] - which could act as a noise deflector and reduce the noise going down to your neigbours flat.

I think the lady must be extra-sensitive and I'm with bunterboy on the g&t idea!

"Does anyone know where I can buy the rubber feet? I can buy in UK or in Italy." An inexpensive solution would be to put a couple of rubber washers [those used for taps] under each 'foot' and screw through them for the fixing