The noise you are hearing could be pump cavitation noises. Which can be caused by sheared (thin) fluid, cheap thin fluid, overheating (thin fluid) and sometimes a worn pump. Sometimes because of fluid starvation, low fluid, blocked filter. Or in the case of 700R4's on two seperate occasions on my truck, blocked cooler lines. Which is a really expensive lesson in the mundane.
Once the cooler line was almost completely blocked, the second time partially.
Compare the length of your dipstick, with another. My rebuilds were complete and sent with a new dipstick. One with the wrong dipstick.
My 700R4's seemed to be good at spitting metal when they were failing, anywhere from an ounce to a spoonful in the pan and filter.
I seem to remember flushing the cooler and cooler lines as being a warranty requirement. Not all that difficult and obviously some engineer thought it was important enough to include in the rebuild warranty.
Low pressures can be a killer in the 700R4, be careful shifting from reverse to forward and the other way around, wait for it to engage before you gas it.