Welcome! Here are the website rules, as well as some tips for using this forum.
Do I even have main vents, let alone sufficient ones?
I've been doing some reading on this wall about the importance of having fast main vents. And while I do see a couple of vents near my boiler (the Vent Rite 33s pictured), I'm not sure whether these are what's being talked about when people talk about main vents, and was hoping someone might be able to help me get a better understanding on that. Those Vent Rites aren't on particularly large pipes, and I'm not really sure what the pipes they're on do — they disappear into a wall never to be seen again. I've also attached a second photo showing how the pipes the vents are connected to attach to the boiler down below.
The reason I even looked at the piping around the boiler is because the (adjustable) air vent on my bedroom radiator seems intent on hissing and whistling and waking me up. I was considering getting a new vent for that radiator to shut it up, but don't want to have the new one scream at me, too. So I wondered if it's whistling because there's too much pressure in the system, maybe because insufficient or non-existent main venting?
Any insight would be much appreciated.