Air in central heating system

[Bern8]

Looking for advice,
Central Heating system is a conventional boiler with an immersion. Think it is configured as a Y system.
At present I am not using the rads (summer). The timer is set to heat the water for 1hr in the morning & same in the evening. When the heating kicks in to heat the water in the immersion it makes a rattling noise as though air is being pushed through the pipes. Then when that settles down it sounds like air gurgling.
Boiler is an old Baxi boiler and just has a dial from 0 to 4, I have it at around 1.5. I’ve never touched the stat on the immersion.
Any suggestions? Cheers.

 

[Johntheo5]

You will sometimes have a manual or automatic air vent installed where the hot pipe from the boiler enters the cylinder coil (top connection), if not, you can slacken the compression nut say 3/4 of a turn (boiler off) and pull back on the pipe to vent any air that may be trapped.