I had a similar setup originally on my closed loops and finally convinced myself that the MAG pump impellers were cavetating(sp?).

If possible, lower the elevation of the pump to a level below the tank, thus increasing the head pressure on the intake side of the pump. If they are cavetating, that should reduce/stop the cavetation. If so, the solution is to relocate your pumps, or replace the pumps. I didn't have the option to lower my pumps, so I replaced the MAGs with Blueline pumps. I also tried to increase the inlet pipe diameter, which helped a little, but didn't cure the problem completely.

There is also a chance you have an air leak along the pipe? Turn off the pump for a few hours and look for any drips at the PVC joints - it doesn't take much!