ruthieandjohn
Stumbling towards enlightenment
(Formerly known as kayandjohn.)
It was most noticeable with audiobooks so I did send them back. Though if yours don't have the same problem, I might go for another pair as it sounds as though I had a faulty set.
Thanks for everyone's feedback.
For the audiobooks, I wonder if there was the same background hiss when you used a corded headphone instead of Bluetooth? I can imagine that since audiobooks began as cassettes, they might of just been copied, as high-noise cassettes, into the file you were listening to. That would cause the problem of hiss independent of Bluetooth.