Hey Chris...that '34 model is beautiful! What's the refrigerant? Is it R-114 or R-12?
Latest development that has me somewhat puzzled: I let the unit sit for about a week and turned it on again - it still exhibited the knocking / hammering sound, so I shut it down. On a whim, a day or two later, I turned it on and it ran perfectly quiet, just like it did when I got it(?!?) It ran through three on/off cycles over about an hour’s time, then the noise returned again. Oddly, it did not come back while it was running – it was humming along quietly, kicked off, then was knocking immediately when it kicked back in again. So, I'm totally baffled. The only thing I can think of is maybe the screen on the receiver end of the liquid line is sucking up some rust or other crud. Liquid flows up that line even when the compressor is off, as soon as the float in the evaporator calls for more liquid. This restriction is, perhaps, causing an abnormally high pressure in the receiver, which is in turn raising the head pressure and causing the knock? Obviously some liquid is getting through, since it does not seem to adversely impact its ability to cool – the coil is still freezing cold. But, whenever it’s knocking, the head and discharge line from the compressor feel somewhat hotter than I think they should, so that would somewhat make sense. I’m guessing that, eventually, the crap fell off and allowed it to briefly run normally again before it again was sucked up? Regardless, I’m now thinking that maybe air has NOT gotten in after all. I really hate to take the caps off of the service valves and start turning them or it WILL definitely start leaking. But, then again, it isn’t very useful as it stands either. Maybe Travis has some ideas?