Hello!. First time here and looking for help. I have a Worcester gas fired boiler (4 yrs old) located downstairs, 7 rads, unsure of pump type. At initial startup, I get a rushing noise from the pump and de aerator which ceases after about 5 mins. 2 upstairs rads need daily bleeding, air has a strong metallic smell, water is dirty grey. Expansion tank has clear water, but there are pieces of rust at the bottom. Plumber who fitted boiler was Corgi Reg and added Sentinel X1100 (?) to system. Water and rads get plenty hot. From reading various forums I think that I may have corrosion issues?. 
Any advice gratefully received.
Any advice gratefully received.