thanks for the reply. the noise is infrequent and sounds like a small pump working hard for a few seconds to 30-40 seconds. i've opened the hood and it is definitely the brake system/modulator. the abs indicator light comes on but i have not felt any decline in brake performance.
since my last post i have done some more research and found the correct part on line for about $520. driving down the road the other night i came across a wrecking yard called Woodfin located in midlothian virginia. they had guaranteed used part for $275 and a guy next door who will install for $100. heres the link www.woodfins.com.