Why would an old return line start hammering after a new boiler install?

I went through almost this exact thing after swapping my old boiler two winters back. Hammering showed up about a week after the new one fired, worst at the end of the cycle, and a return joint down in the basement started weeping. Here's what it turned out to be for me and what I'd bet is happening with yours.

A new boiler is full of oil and casting junk from the factory. All that stuff floats on the water surface and makes the water foam and surge up into the header, which throws slugs of water into the piping. That's your hammer. The fix is skimming, and not a quick five-minute skim. It's a slow process where you open the skim tap and let the boiler boil gently while water dribbles off the top surface for an hour or more, sometimes across a couple of sessions. Ask your installer straight up whether they skimmed it and for how long. If they say they didn't, that's your answer right there. The second thing to check is the near-boiler piping itself. If they rebuilt the header, the risers and the takeoffs need to be sized and spaced right so condensation drains back instead of getting shoved around. A header done wrong will hammer forever no matter how clean the water is. Take some pictures of the piping above the boiler and post them here, people will spot problems fast. As for the dripping joint, hammering is probly what's working it loose. Old return lines that sat quietly for decades don't love getting shaken. But stop topping off the water. If the feeder is kicking in that often you're either feeding the leak or losing steam somewhere, and constantly adding fresh water brings in oxygen and minerals that corrode the boiler from the inside. Mark the waterline, note how fast it drops, and let the installer see it. That info will help them figure out where the water is going. For me it settled down completly after a proper skim. Fingers crossed yours is the same.

Same thing happened to my dad's place after his swap, but it wasn't the water, it was the old return itself. Decades of rust and sludge were sitting in that line doing nothing, then the new boiler's got more push and it all packed into a low spot. Return hammered every cycle til a section of it plugged solid enough that water couldn't get back, and the boiler started bitching about low water. Plumber cut out the worst four feet and it's been quiet since. So yeah, don't assume it's all boiler. If that return's original, some of it might just be done.

Check the air vents on your radiators before you blame the returns. When I did my uncle's swap last spring his feeder was kicking on constantly and it turned out the vent on the end-of-line radiator was spitting a good cup of water per cycle. All that water going out the vent has to get replaced by the feeder, so it looks exactly like a leak down in the basement. Feel the floors under each vent too, a spitting vent leaves a little wet ring. If the vents are dry then yeah, the drip joint is probably your water loss. Either way I'd flip the feeder to manual or off untill the installer shows up, one day of low water won't hurt anything.

This is probably a dumb question but what does the banging actually sound like? Ours after the new furnace guy put in a boiler was more of a knocking at the ceiling than a real hammer, and it turned out one of the pipes up there had shifted and was touching a floor joist. Metal expanding and rubbing on wood makes a racket that sounds just like water problems. A bit of padding between the pipe and the joist shut it right up. Might be worth walking the basement while it's running and listening for where the noise is loudest before the installer shows up.