I wouldn't worry about air sounds in the system if pressure isn't going down.

Are all your rads filled from the bottom? We have had to top fill one of our rads and it makes a bit of noise, probably as it has a small air gap at the top.

A you-tuber I follow reccommends loading up your system with a bottle or two of cheap strong perfume, if you catch a whiff of it around the house then you'll likely have a leak around that area.