That noise could be the pump being dry for a second whilst the water is going to all the right places once the system has no air leaks in it after you draining it and filling it up again.
You have left the header tank cap off whilst running the car once you have re-filled it for at least ten minutes to clear the air through the system?
Give the return hose a few squezes too whilst this is being done.

It could be your water pump is on its way out too. They are still available at some place's for the 4x4 system but are pricey. £185 I paid for mine from woodford garage in Totnes a few years ago. I'm sure they do overseas postage.

Also I personally wouldnt fill the header to the top, but 3/4 because, in my none mechanical mind lol, your not leaving enough room for the air that Is left in the system to escape to in the header tank.

Just a few ideas but hope this might help.