I'm just wondering if the OP's problem is the ABS pump unit reservoir is having issues. It is basically a spring loaded cannister with a diaphragm, If it has issues it usually will make a groaning sound due to fluids in a faulty unit. Kind of like a ships hull when it is sinking and under hydraulic pressure. It will groan. When you have 5 trucks, a car, a motorhome, two boats and a jetski, you learn stuff. WTF was I thinking?