5 days post op Breast Reduction, I just noticed this morning like a hollow/swooshing sound when I tap my breast. Any suggestion?

It certainly will take time to resolve.  It may not return to normal, but it is not possible to predict what is going to happen this early after surgery.  Be patient.

After breast reduction, you should be in your house doing nothing for a week.  After that you can go back to work.  No exercise for 1 month.  By 6 weeks most people are able to do cardio and will feel very well.  You should be able to go on holiday.

It is not really possible to change the breast for FTM without surgery.  You need to find a surgeon that specializes in the procedure and speak to them.  At your young age they will require that you speak to a counselor to help you make decisions.

You may be doing too much in terms of physical activity this soon after surgery.  If you do something that results in swelling, stop.

This is a problem we face sometimes when the areola is simply too large to removed in one operation. However, after you have healed and the skin has stretched and softened, a revision can be done without problem and minimal down time. See the link below for an example of this phenomenon.

In my opinion, assuming your breasts healed as expected, at 8 weeks postop from breast reduction surgery, you should be back to your baseline energy. Best to make an appointment with your primary care physician for an exam to make sure there is no underlying health problem.