Q:

I'll be gone in a whistle. What am I?

I can look through like superman,
and could even save your life.
I can fly faster than a jet,
but I'm not out of this world
You won't even know I'm there,
so frequently,
I'll be gone in a whistle.

What am I?

A:

Could it be

 Sound?

I can look through like superman, and could even save your life.

 Ultrasound can produce a sonogram - useful in medical applications

I can fly faster than a jet, but I'm not out of this world

 Sound waves move faster through air than many jets, but do not travel where there is no medium (a vacuum)

You won't even know I'm there

 In most cases, you cannot see sound waves

so frequently, I'll be gone in a whistle.

 This one, I'm not sure about. I thought, perhaps, the Doppler effect, but that would make a higher frequency while approaching rather than receding. 

A:

Made me think of

 X-Rays  

I can look through like superman,
and could even save your life.

Used to scan your body

I can fly faster than a jet,
but I'm not out of this world

 Both frequency and speed of x-rays are extremely high. Yet they are nothing imaginary or fantastical (like superman)

You won't even know I'm there,

 X-Rays are not visable to the human eye

so frequently,
I'll be gone in a whistle.

 A hint towards waves ;) ?

A:

I think the answer is

 x-rays

I can look through like superman,

 x-rays are used to view inside the human body and other solid objects

and could even save your life.

 x-rays can detect things like internal injuries

I can fly faster than a jet,

 x-rays move at the speed of light

but I'm not out of this world

 very common to find x-rays in use

You won't even know I'm there, 

 can't see, smell, taste, feel, or hear x-rays

so frequently,
I'll be gone in a whistle.

 not sure on this one...

Maybe I'm thinking way too literally

