Ok, I found what is the reason for my problem. After I inspected the hoses I found out that there is a vacuum leak from the valve cover hose #13, right where the connection is.
http://www.realoem.com/bmw/showparts...62&hg=11&fg=15 I guess that when they changed my valve cover gasket, they had a hard time removing the hose from the cover and they damaged it. Additional to this they did't put a hose clamp back to keep it tight and secure.
I will use hose tape and a new hose clamp to fix the problem, because if have to change it, I have to involve the second hose which is a part of the cooling system and it's gonna get messy.