Check your rear spoiler, assuming your mustang has one.
I noticed a sloshing sound coming from the trunk of my 03 Mach1 last year. Soon after this, I thought I heard air leaking from a tire. It turned out to be air leaking from the spoiler.
After removing the spoiler, it appeared to have about 1/2 a gallon of water inside it. I could not see any holes in it, but I assume there must be one or more small pin holes in it somewhere.
Even with the spoiler removed, the water would not drain from it. I ended up drilling a small hole on each side of the spoiler where it mounts to the trunk.