During Hong-Sau meditation I often hear a delicate "ringing" in my ears. It is a relaxing sound, but I don't feel any special bliss associated to it.
Should I try to concentrate on this sound, or just ignore it?
The sound you hear might simply be a more-or-less normal sound of the physical body, but it might also be a chakra sound.
If it’s a body sound, yes, definitely ignore it. If it’s a chakra sound, you’ll have to decide whether to continue with Hong-Sau (in which case, ignore the sound), or stop Hong-Sau and concentrate on the sound. How to decide? Trial and error, noticing which course of action brings a greater sense of upliftment.
How can you know which type of sound it is? Since you say that the sound is relaxing, I suggest an experiment: Stop practicing Hong-Sau, and listen intently to the sound. Absorb yourself in it, and notice the results. Is it just relaxing – never something more – or does it bring a deep sense of peace or another divine quality, such as joy or love? If it’s a true chakra sound (i.e., a manifestation of AUM), it should be more than just relaxing.
In any case, hearing that sound is not a bad thing. Just make a conscious decision as to what you do, rather than trying to do two things at once.
Blessings on your practice,