A friend of mine is living in a place where, almost under her window, several busses wait for passengers with their air-conditioning compressors running, generating lots of noise, from 5AM onwards.

Soundproofing her apartment would be difficult, due to some circumstances.

Does someone knows of an "ambient noise supressor", operating with a similar technology as noise cancelling earphones/headphones?

So called white noise generators would be of no help.

I understand that such an appliance would probably be much less effective than a headphone/earphone, if at all possible, but would like to try it anyway.