Fiser: Try doing what I did. Near the back/side of the the toilet, there is a valve which you can tighten/loosen. This will regulate the amount of water that goes into the toilet. If you perhaps tighten it and then loosen it, you can get a sense of (when you flush) if the noise is still there. You may have to flush a few times and adjust this valve to get an idea.
Personally, I have to experiment a bit. My baby is asleep when I usually get home, so I'll have to wait until the weekend...
Scientifically though, i wonder is this pressure that's causing it?
I wish I took courses in physics!!