Removing bladder tank solved pipe banging problem...why???

I have had repeated banging in my pipes for the last few months. It started suddenly and I was unable to find any valves, toilet valves, etc that had any issues. I moved one of the bladder tanks (I have one on each of two water heaters on each end of the house) to where the water comes into the house thinking the pulses might be coming from the water meter/check valve. This just changed the frequency slightly (around 3 per second). It would repeat for 30 seconds sometimes. I discovered that turning off the inlet to one water heater stopped the noise but turning off the outlet valve didn't. I could find nothing wrong with the tank. The only other thing there was the bladder tank so I took it out and plugged the spot. The noise is gone! It seems to be some oscillation between the bladder tanks. I would like to understand what was happening. Anyone got any ideas?

There was no "oscillation" between the tanks which could cause any sound. IF you have a check valve installed, then pressure changes could cause the expansion tank(s) to make it create a noise like that. But only an onsite inspection could tell what was the actual cause of the sounds.