I discovered that it does have something to do with the ABS modulator. The gas chamber next to the resevoir vibrates when the noise activates, then I can see fluid move in the resevoir. The resevoir level is also low.

 

I checked the codes and topped off the fluid.
With the codes here is what I found:
13 - In the shop manal it suggest a High pressure leakage, leaking outlet valve, leaking releif valve or poor connetion in pressure switch current.
18 - Shop manual suggests Hight pressure system malfunction, Accumulator gas leakage, changed releif valve set pressure, rear outlet solenoid valve late to close, changed pressure switch set pressure.

Trouble shooting guide suggests replacing the modulator for just about all of the symptoms.
After inspecting the area around the modulator I see no indication of leakage except for where I spilled some after topping off the resevoir.

 

If fluid level was low, I would try to find the source and see if a fix is possible. The internal leaks are not repairable. I worked w/ a modulator w/ an internal leak for several months thinking I might be able to fix. Iremoved valves, cleaned,disassembled, inspected, and reassembled w/ no improvement. I think the actual culprit is the small check valve that allows the tiny pump to pressurize the accumulator. Even a tiny leak by this check valve will result in eventual low pressure and pump will reengage to restore accumulator pressure.

I've found used modulators for prices from $25-$100 easily w/i my immediate area on ebay or car-part.com. If you buy from a parts yard insist on a warranty as you may get a non-working part. They have no way of knowing if a part is good or bad when they sell it, but most will offer a replacement unit or money back for a non-working part. On my daughter's 97 Acural 2.2 CL, I had to mate the pump motor from the old unit to the modulator body and accumulator from a junk yard part which had a non-working motor.

good luck