I don't think it has much to do with any Physiological problems, and it's not as troubling as you might think. The point is that Astral Hearing is Activating before sleep or in Trance, which is a good indicator. Taking into account your remark that you had nothing like this before the Initiation, and that your Health is normal, then, most likely, this is a consequence of your Spiritual Advancement, in particular, of working with Astral Senses. You are catching completely random Vibrations happening around you. I had exactly the same situation.
This whistling is Astral Phonic Noise, Vibrations of surrounding objects, because every object in the Universe Vibrates. In cities there is a lot of this Astral noises, and the Astral Ears are perfectly capable of perceiving it, although it is a stumbling Block for many Beginners when they first encounter this Effect, which, although positive, but not always pleasant, especially if you live in a metropolis, as it is often difficult to Concentrate and work Spiritually.
The Stronger the Astral Ears Open, the clearer the whistling, but I really advise you not to Focus on this Noise for long, otherwise it will become unpleasant. In the countryside or out of big cities such whistling is minimized and just there the beauty of the sounds of Nature opens up, the Vibrations of which on the Astral are much more harmonious and much less sharp, they are «soft».
As your Spiritual Development progresses you will find it easier to control this. As long as it happens a bit randomly, don't worry about it. In the meantime, I recommend that you continue to work with your Minor Astral Chakras and Develop your Psychic Sensors. Subsequently, the whistling will be transformed to the full interpretation of all Astral Signals by your «Transformers», which transmit and perceive these Noises and any other Signals from the Fourth Dimension (i. e. Hypothalamus, Hypophysis and Pineal Gland)
