Noisy neighbours!

Hi, we live in a mid terrace but are fortunate to have a lovely bird friendly garden. We love feeding the birds and watching the countless goldfinches, blue tits and sparrows visiting. 

Sadly about 4-4 days ago, our neighbours have taken it on themselves to purchase a device that makes an oscillating high pitched noise and have positioned it pointing at our feeders!! Since this has happened - no birds! We can also hear this noise!!! Very frustrating.

We are incandescent! But want to maintain good relations with our neighbours also. ( they may be allergic to birds??). Any advice please?