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

A swooshing sound is most likely fluid.  I always use drains because fluid will build up in this space.  You might need a drainage.  Go see your plastic surgeon.

Thank you for your question. There may be a small amount of fluid in the area and this should absorb over a few days. Notify your surgon of this and ask them if they would like to evaluate you. I hope this helps!

Exam is needed to evaluate for fluid
collection. Best to make an appointment in the next couple of days to be
examined. Having said that, this will likely be of no significant consequence
in the long-term outcome.

A swooshing sound may indicate fluid in the breast.  Best to be seen by your surgeon to be properly evaluated, it may need to be drained.