when I press the octave key there is a strong whistling sound and when I play the mid register there is always a bit of this sound in the background. Very annoying. Do I have a leak? it is ok when I play the low register. I’m sure it is not my embouchure.
Hard to say without experiencing it but sounds like your assumption is probably right. If it happens every single time it’s the horn but if not then it’s probably you.