First, verify that the system water pressure is correct.
The system should have an air extractor...some work better than others, and as they get older, they can get plugged up with mineral deposits. But, other than that, it's easiest to extract air if there are purge valves at the highest points. Have you checked if there are any valves behind the covers? You need to open those valves, and have the autofill valve open for it to be able to fill the volume vacated by the air with fresh water. If not, you need to use the highest drain valve and run pressurized water (from the fill valve) into the system until it runs clear without bubbles. You may need to do this several times as the new, fresh water will have some dissolved air in it.