Yes, it somewhat reduces intake noise. It is not indispensible, but it would be hard to modify for an expansion tank as it is connected to the intake manifold. It would be a pain to seal and it would cause catastrophic failure (water in cylinder) if the seal ever fails. I have ziptied a 50 ml flask to my radiator which acts as a halfway proper expansion tank. It regularly overflows, though, I guess something in the range of 100-200 ml would be better. Also, running without an expansion tank is not out of question. First time water expands, some of it pours out, when it contracts, some of the radiator remains empty. But after all, when water expands again, it fills the radiator. I don't see the problem in this, unless most of the water is somehow removed from the system. A lot of KTMs run like this. Someone please correct me if I am wrong?