Hello, as you state 2 facts. "Rattling" and "Noise stops when turn low"
I do not think there is air is the system. The problem lays with a faulty valve just as Browns (above) has stated. 99% sure as seen this many times over 30 years. This also happens with noisy tap pipes. Change the spline and all is quiet. So change your TRV and all will work. You have answered your own question by saying turning it down reduces or stops the noise. This restriction of waterflow is your evidence. Nothing to do with Air. Expect to pay 80 to 150 depending where you live and what system you have and where the rad is. Upstairs or downstairs as only need drain half if upstairs. Make sure your chap adds inhibitor when he fills up.

Entire system needs bleeding.
Cause of the noise is due to air bubbles travelling trough the radiator.
All air need to be out .

Hi Kim. Sounds like air in the system. One question to seek out is how did it get in? The other is how to remove it?
A conventional tank / gravity system can draw air in if the pump is set to high but these systems are also easy to bleed air from.
A combi boiler does not draw air in, they can of course be air bleed but will need to be carefully filled back up, preferably undertaken by an plumbing / heating engineer.
Good luck, Bob.

As with the two others posts, air is one of the most common causes of noise in central heating systems but if it is quite loud this could also be caused by a chattering thermostatic radiator valve.