For something like this an ultrasonic leak detector would likely be ideal. Much like wind blowing through an open window, as air flows through the leak it makes

For something like this an ultrasonic leak detector would likely be ideal. Much like wind blowing through an open window, as air flows through the leak it makes noise which is totally inaudible to the human ear but detectable by sensitive electronics. The closer you get to the leak, the louder it is.