As Ben noted, if there's any air in that reservoir, it's too small to avoid it getting sucked into the pump inlet. Sure, adding a restriction and reducing flow will probably eliminate the problem because the velocity through the reservoir will slow. This is not the same as cavitation, either. Cavitation is lowering the pressure of the fluid so much that some of it vaporizes. These "bubbles" tend to implode as they get to the higher pressure present near the pump outlet. This effect can literally eat away impellers (even the metal ones in industrial pumps). The pressure is lowered at the inlet as a combination of vacuum and fluid velocity. High velocity equals low static pressure (same idea that keeps planes in the air).
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