I noticed that the noise stops for a few minutes. Someone told me that this is due to the pressure of the fluid. So this must be normal for the unit. I think it would be a waste of time trying to contact with the support of Silverstone. I say this, because my APC unit has a few problems that shouldn't have and I got a new one and it had the same problem, except one which has been fixed, as the unit was almost a year older than mine. Unfortunately, this unit had an always on fan and for this reason I returned it back. So I don't think that I should do the same mistake again. The Tundra has 2 years of warranty if I'm not mistaken, so I don't have to worry about it.
Thanks for your help anyway.