Do I even have main vents, let alone sufficient ones?

Many thanks, everyone. @gfrbrookline , it's a one pipe system, and the diameter of the main (as calculated from the outer circumference) is 2.4" from outer edge to edge. When measuring the length of the main, do I measure the entirety of it's horizontal loop around the basement/garage, from where smaller pipes come out vertically up to the radiators? If so, it's roughly an 80' circuit.

@JUGHNE , the two pipes do tie together on the way down to the boiler. I'll attach a hopefully better photo of that in a separate comment below.

Thanks @JUGHNE. I'm not the most adept at this kind of stuff. Is this what it's supposed to look like at the end of the process you describe (with the same setup also on the second dry return)?

@gfrbrookline, the dry returns, and some of the main, disappear into the wall and above the bathroom ceiling. I can try to pop up the drop ceiling for a more precise measurement. Are the precise measurements needed to figure out the size of the main vents?

So, I have an update to the above (which doubles as a request for updated thoughts from the Wall, @New England SteamWorks , @JUGHNE et al):

I had hoped to get New England SteamWorks to come look at my system, install a couple of big mouth's I bought (the bushing connecting those small main vents was stuck on there really good, so I couldn't do it myself) and to help me eliminate the wake-up whistle from my bedroom rad vent and a new clanking sound from another bedroom radiator. But those guys seem to be in very high demand and haven't been able to schedule a visit to my part of town.

So I got another plumber/heat worker in here, who emphatically told me that my big mouths aren't actually vents, but traps. Since he's the plumber and I can't pretend to know much about heat, I didn't feel like I was in position to argue. He also suggested that the whistling and clanking was because the ventrite #1s on all my radiators weren't wide open.

So, after splitting the bushing in half trying to remove them (told you they were stuck on there really good!) he eventually replaced the vents pictured above with Hoffman #1s -- at the same location where the current vents are, and not up and back, as suggested above. And he opened the adjustable Ventrites all the way all up on every radiator throughout the house.

Again, it didn't feel like I was in a position to suggest otherwise. He's the plumber, I'm someone who's read an internet forum. He certainly knows more than me, and I don't doubt that there may be a reason for his decisions.

But I do wonder what people's thoughts are. I've read again and again that the mains should be vented fast and the radiators vented slowly. Are Big Mouths main vents and not steam traps? (Do they just look like steam traps)? The #1s are, I assume substantially slower than Big Mouths. Are they fast enough? And is indeed there any reason it might be best to have the setting on the adjustable ventrites open all the way throughout the house? He suggested, if I remember correctly, that by turning them down I was creating too much pressure in there, particularly if my boiler is too big, which he speculated was possible. Maybe in my setup wide open is best?

At any rate, the whistling hasn't gone away, and the clanking hasn't either.

Any thoughts? If everything is as it should be now, I'd certainly like to try to return those pricey big mouths.