I'm a beginning flute player. When I play, I often- but not always- have a "shh" sound along with it. I noticed that it usually starts out pretty quiet when I start practicing, but after I play for about an hour it gets totally out of control. And since I usually play for an hour or more in one setting, that is a major problem. I've tried changing my lip shape and tilting my flute back and forth, and I found there's a spot where it doesn't sound as bad as the other spots, but the shh sound almost never fully goes away. Anybody know how to help with this?
I know people say keep practising, but that's half of your problem! As a beginner, it's difficult to keep that perfect embouchure for an hour, and really, at your stage, it's becoming non-productive - as you found out.
Keep the practice sessions shorter - perhaps 20 to 30 mins at a time. Leave a good hour in between, to give your mouth muscles time to recover. You need that time, and the results will improve because of it. or go even more, and play for 15 mins, but 4 times a day. Note this is not necessarily a permanent situation, as, when your embouchure gets stronger - as in it can be maintained for longer - so your playing sessions can and will lengthen.
As Tim Burnett - Bassist said, the problem may be that you're getting tired as you play (you mentioned being a beginner and playing for over an hour - that's rough!). Even advanced students and professionals start to sound pretty bad when they're exhausted and their emboucheres are relaxing. It's generally a sign you should stop.
Definitely the best advice you can have is practice! I remember that infuriating hissing sound - sometimes it seemed like it was more hiss than note - and wondering why I couldn't have a nice pretty tone like my teacher's. Tone studies (where you hold each note in turn until you can get it to a nice, full, round tone) definitely helped me; you can talk to your teacher about what tone study would work best for you. :)
Check out this thread on fluteland.com. Notably, it's mentioned that this is a 'by-individual-musician' issue...
Also, this nice gentleman is beautifully demonstrating body position, fingering, etc. on a wooden flute with no hiss:
Your ultimate answer here, as you may find in other places on this SE, is: practice, practice, practice. As play becomes second nature, and your instrument becomes an extension of your body, this problem will likely solve itself. Keep it up!
Adding onto other answers regarding embouchure, when playing for extended periods of time, a high amount of moisture from the air you blow into the flute develops inside the opening and pools into droplets inside the head of the instrument. Try swabbing out the instrument with a flag or with a cloth and stick (the kind where you thread the cloth into the stick to be able to push/pull/turn). You might notice some decrease in hiss after that.