Thanks for the reply. I'll add the additional water tonight. The pumps are indeed installed below the level of the tank. They run fine and quiet until the tank empties then the noise starts, which is why I assumed it was sucking air and shut it off immediately. I'll add the water tonight until it drains out of the P&T relief valve, and try again. How much water should be visible in the sight glass when the pump is running (so I know the heat exchanger coil is adequately covered)?
1. You do not have enough water in the system. Rather than filling the tank to within 1/2" of the top, you should open up the relief valve in it, and fill it until water is coming out of the valve, to make sure it is completely filled. Then close the valve.
2. Each panel generally holds about a gallon of water, so you would have 3 gallons total in them. With 76 feet of 3/4" pipe, at 0.02227 gallons per foot (assuming type K), that's 1.7 gallons in the tank. So that's about 5 gallons between collectors and pipes.
3. (OK, you didn't have a #3, but I'm adding it). You want to make sure the pump is never, ever dry. It should be installed well below the drainback tank, so even when the pump turns off and all of the water rushes back in, it doesn't have a chance to ever add air into the pump and potentially create an air lock.
Hope this helps.
Solar Thermal Queen