As one person I saw already said, likely cavitation.
This happens when the pressure differential between the inlet of the pump and the outlet of the pump is too large. This means either too much back pressure, or not enough inlet pressure.
Making sure the fluid level is high, there is no blockage from the reservoir to the inlet, no leaks on the lower pressure side, no restrictions (coolers that wind back and fourth across aluminum fins are BAD, you need a straight through cooler).
Your reservoir needs to be the highest point in the system and the path back to it from the RAM / gearbox needs to all be uphill or else it wont bleed well.