Help with noisy baseboard heat.

I moved into a 40-yr old home last summer. Last winter, the baseboard heat made the common clink-clank sound as the pipes expanded every time the heat went on. I've come to the point where I've accepted that I'll always have that.
But this year, my upstairs unit sounds like Niagara falls when the heat goes on. I understand that means there is air in the line. But why? Where did the water go? Is it likely that I have a leak somewhere that is slowly damaging my home and I just can't see it?
Furthermore, regarding solutions. I've read many posts about bleeding air out of the valves in each section of the baseboard units. But I've checked each piece and there are no bleed valves on the piping. However, down near the furnace, there are two on the outbound line, and one on the return. Furthermore, there is a pressure reducing valve followed by a regular manual valve that, I suppose, allows water into the system.
So, should I open one of those spigots to flush the system? Which one? I'm a little leary as it seems that will just drain the entire system and I'm not convinced it will eliminate all the air.
Please help. The sound of rushing water in my walls is very unsettling. Thanks.

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