Which gears not to use in a 2 X 9 gear bike I am new to biking. I bought a hybrid bike with 9 gears at the back tire and 2 gears at the front. For reference lets say gears at front (chain ring) are numbered 1-2 and gears at back are numbered 1-9. As the numbers increase the the cycle becomes harder to pedal (for both front and back). For example if you switch from 1 to 2 at front cycle is harder to pedal. And if you switch from 2 to 3 at back tire, the cycle is harder to pedal. 

Now my question is this. Are there certain gear combinations, I should not be using. Can you write those combinations down for me like (1 back, 5 front). So I can use that for reference. The particular problem I am having is that when I am at around 7-9 at back and 1 or 2 at front the cycle starts making a weird noise. Is it normal? Any other combination I should not use?
The general rule is that you don't want the chain angle to be extreme (i.e. the chain should be running as parallel to the frame as possible) - when the chain is at an extreme angle, it is called cross chaining. This is a good read. 

For example, you don't want to use the little ring in the front and the little cog in the back (this increases wear on both of them). Likewise, if you want to use the big ring in the front and big ring in the back, you may want to go to the middle ring in the front instead and remain near the center in the rear (if you had it -- for a double in the front, there will be a point where you'll prefer to shift down in the front and then stay near the middle of the cassette). 

The most important thing to do is to avoid small-small combination though - modern drivetrains aren't too unhappy with cross chaining, but its still not good practice. 

Also, note that the gears you use at the time depend on the terrain and what not -- try to aim for a cadence of about 70-90 RPM (how fast the pedals turn).