Air in cold water supply - please advise, thanks!?

Hi, I have noticed in the last few days that when I open my kitchen cold water tap after it has not been used for a while, there is a brief spurt of air before the water runs. It is only for about half a second or so, but my guess is there should not be any air in there at all! Our kitchen tap is a mixer tap - we have no problem with hot water. Other taps (including an outside tap) seem OK - it does seem to be only the kitchen tap that is affected , but the water pressure is much better donwstairs than it is upstairs. We have not needed to "shut down" the water supply (unless the water authorities have done this via the main tap outside my house for some reason). A previous plumber scared us a little when he told us we had steel pipes (this is apparently due to the fact that copper was in short supply when our house was built in the early 70's) and he told us that any air in the system could cause the pipes to rust / corrode. We do not hear any noises that suggest the cold water supply is constantly refilling, so I am assuming no leaks anywhere in the sytem. Is there a quick way that I can solve this problem or does it suggest a bigger problem that needs a visit from a plumber? Thanks to anyone who replies!!