There is no back up in sink or shower. Nothing using water is turned on. Flush handle is sometimes hard to press. Why?
On the other hand, if you mean a hissing sound, that's probably a float valve not completely closing.
Metallic flush handles are sometimes hard to press after they corrode a bit or after excessive use; they form a catch that's been worn into their housing (plastic levers are less susceptible to this).
By air noises do you mean bubbling? If so, it could be that your drain vent is blocked -- possibly by a small animal that crawled in through the roof and died. That was my experience recently. When the vent is blocked gas will find other ways to escape the system. Unfortunately, that means into your home. In your case, it could mean through the toilet.
This sounds like a job for a plumber, who may need to get onto your roof and go in with a snake.