Yes, you need a radiator key, which you can get at any hardware store, or if you have nice neighbours from next door. You then loosen the valve at the top of the radiator and allow the air out (have a towel handy and wedge it behind the radiator as water will spurt out when all the air is out). If this doesn't work the problem will be elsewhere try bleeding and refilling the system. The bleeder valve is usually found at the lowest point in the system, usually just under the boiler and then refill. Failing this call a plumber.