Some baseboard heaters not working while others are in same zone

Other than looking for hidden bleeders (i.e. I have one where the baseboard cover needs to be removed to reveal the bleeder) and bleeding any bleeders that you can find, you may need to swear at whoever put in the new radiators and retrofit bleeders where they are missing.

There is often an automatic bleeder/vent on or just after (in which case it's often on a big cast iron "air separator" that may have an expansion tank hung off its bottom) the boiler, since in theory any dissolved air will be most prone to come out of solution where it's hottest - if that becomes clogged or otherwise inoperable (they don't last overly long) air may migrate elsewhere more than it would if that was working. Sometimes people see a dribble from these and close down the cap on the "tire fill valve" rather than replacing them - which renders them inoperable. I think it's bad practice to depend on that for bleeding the whole system.

Given how anemic circulator pumps are when faced with air bubbles, the only other way of getting that loop clear would be to open the boiler fill and open a drain on that loop, so that the fill water can push the bubble through to the drain - but then you have new water in the system, and some air will come out of that.

Beside air, could anything else be causing the noise?

Depends on exactly what the noise is, but I would be on the lookout for thermal expansion. If you have long, straight runs, the pipe can expand significantly when heat is delivered. If there are places where the pipe binds on the supports, it can make pinging and popping noises as it alternately binds and frees itself. If it's attached to a loose metal airflow director, it can rattle.

Because this noise isn't coming from inside the pipe, pulling or shifting the pipe as you turn it on will change the sound significantly, but shouldn't make much of a difference if there is air inside.