Hoping to get some guidance.

I just re-connected my basement heat zone (#3). When circulating water, the flow/check valve, a Watts Model 2000S, rattles. The sound stops if I turn the thumb screw counter clockwise a half turn or so. But I'm pretty sure that is not the solution.

Can you advise me what the problem might be? Should I rebuilt it? Could there be air in the system? I bled the system, but it is interesting to me because the heating elements (baseboards) are at ground level, and thus the lowest point in the system. There is a release valve on the intake side at the highest point, but I could see how air could get trapped at the check valve.

Any help is appreciated.

Thanks,
Vin

Vin