Ok, I drove my car with it's LT1 swap today, and noticed a faint whining sound when acceleration.

No, not coming from the radio, it's coming from the engine...

Is this a vacuum leak or something? There's not a single performance problem at all.

Also, when I turn off the engine, I hear a slight hisssssssssss that fades away within 4 seconds.

Sounds like a vacuum leak to me, but I suppose that could also just be the compression blowing off of one cylinder that ended up on it's compression stroke when the engine was shutoff.. Run the engine, shut it off and try to pinpoint the hiss. Vacuum leak or not, it should lead you to the problem.

are you sure the whining isnt your diff about to blow?

are you sure the whining isnt your diff about to blow?
"diff"?

Diff = differential.

No, that wouldn't be the noise.