Severe water hammer and where is my main vent?

Hi all,

I bought a house back in April and have had the fortunate experience of trying to figure out why my steam system sounds like WWIII every time the heat starts up. I've purchased 'We Got Steam Heat' and have been attempting to troubleshoot what plumbers/HVAC companies I've hired can't seem to fix.

Here's what I've had done so far or have done myself:
• All radiator vents have been replaced (some more than once)
• Problematic radiators have been pitched
• The entire system has been flushed more than once
• A cleaning chemical was used after the last flush
• A boiler blow down has been completed about once or twice a month
• The boiler room has been insulated, the rest of the piping in the basement has not, because I live in it and need some of the heat. The insulation did not help with the noise at all.

One company I hired did seem to think that the main vent is somewhere in the ceiling above my boiler and said they'd have to break the ceiling to get to it, but they never came back for the job. This does bring me to my main question though, where is my main vent and what is this weird pipe doing that goes nowhere?! I've attached some images to hopefully get some answers. If the main vent is clogged or missing all together I assume this could be the reason for the water hammer. Another weird quirk I've noticed is that every time I do a blow down the water in the sight glass seems to rise a little. It started at around 1/4-1/2 full and is now about 3/4 from the top, not sure if this is related or not.

Any info you have is greatly appreciated, thanks!