Hearing aids are designed to amplify sounds you want to hear like speech and music, as well as other sounds like birds chirping or a baby laughing. However, hearing aids can also amplify sounds that you may find annoying such as the drone of a refrigerator, washing machine, air conditioner, or even the circuit noise of a hearing aid. All of which can give you the perception of a static hiss.
This "hissing" perception is a result of a few things. First, is your residual hearing ability. The likelihood of you hearing this static hiss is greater if you have good low frequency hearing thresholds. Second, is the use of Compression. Compression is a hearing aid feature designed to increase soft speech sounds and decrease loud sounds. Compression Video: https://youtu.be/x0f3ipYijI4 However, when you increase these soft speech sounds it can also have a side effect of increasing very soft sounds that you may not want to hear.
If you do hear this hiss, there are several things you can do about it. First is adapt to it. In some cases, these sounds just take some time to get used to. Second is reducing the overall amplification of your hearing aids. This is not recommended due to the risk of underamplifying speech sounds that you do want to hear. The Third approach is to use a digital hearing aid feature called Expansion. Expansion is kind of like Compression only instead of increasing the amplification of soft sounds, it actually decreases the amplification of soft sounds.
By reducing the amount of amplification given to very soft sounds, without impacting soft level speech, you can often receive the best of both worlds. So if you are having the perception of a static hiss sound when you wear your hearing aids, and you can't get used to it, expansion is the feature you are looking for.