Why am i hearing water in the inside wall of the trunk on the passenger side of my 2007 nissan altima?

You likely have good hearing, and there is water there.
First, where it came from:
Most likely is a repair job by a body shop that is defective.
They may have repaired the damage from a collision cosmetically, but left some holes or gaps so water come come in when it rains, or if you drive through puddles, etc.
Another answer is the car was in a flood, and has water collected from being submerged, at least in that area.
To fix it, you can take it to a body shop, and they can repair this, though it may cost you quite a bit pf money.
You could also take an electric drill with a metal cutting bit of about 1/4 inch, and make a "weep" hole at the lowest point to keep it draining all the time.
I have done both of these fixes many times.
God bless your efforts.

Check the seals on your tail light, side market light, antennae, trunk seal.