I don't know if the sequence of events is important, but here is what happened:
For the first time since having VS pump installed we had a freeze. Since our old freeze protection unit was not bypassed, the low flow of the pump is causing the heater blower to kick on and it cannot be turned off. Our temps were only supposed to be around 32 so I killed power to all the equipment. I think it got down to about 27 degrees, and the following morning I turned it on. There was an unbelievable banging sound coming from the heater. I'm not 100% sure, but I think that was without the heater actually heating.
I let it sit all day thinking ice had gotten in there. It seemed okay later that evening. Ran the heater and everything. Several days later I fired up the heater again, all seemed ok. At that point the valves were set to return water to both the pool and spa, and suction only from skimmer. I then changed the valves for suction/return only to the spa, and the banging came back. I've read posts of people describing banging noises as sounding like gravel, or maybe popping. This sounds like someone is in there with about 8 hammers hitting the sides of the heater as hard as they can! It is physically moving the whole unit!
I found a youtube video of someone working on the exact same heater, with the symptom being banging noises. The fix was a broken plastic diverter just inside the manifold where the PVC unions connect. Could this possibly cause banging that severe? Its hard to imagine steam and cavitation causing noises like I'm hearing. Any help would be greatly appreciated!