Moved into a condo in a 9 story building. Nice comfy toilets that make an awful gulping, or gluging, sound at the end of the flush; quite loud. I notice that the tank is marked 1.6gal, and the flush valve(?) is marked 1.28gal. I have no idea if this difference make a difference, or what the heck is causing the noise, or how to fix it. Any ideas?