There is a recall on this Part. The dealership will probably not tell you at this point that there is one, but if you have a MY 2000 you are more than likely eligible for it. I have a 2000 with the same problem, the pump runs ALL THE TIME and is loud. It seems pretty clear that there is a recall on this pump that has a very straight forward remedy. I don't know why the dealerships and Volvo seem to be hiding this information so far. I have contacted the dealer once, but now that I printed a copy of the actual letter from Volvo with chassis numbers I am going to go back. This is rediculous! Before you try to fix this yourself take a look at the links below and see if your car is applicable. What could potentially happen is that water could enter the pump and cause it to fail, eliminating majority of your braking force. I have had in happen once, scared the hell out of me, dealership said everything was normal and it never happened again. Good Luck and Be sure to follow up on this! Its a large safety issue that the NHTSA is forcing Volvo to correct!
http://www.motorwatch.com/Service/recalls_upcoming.htm
http://152.122.48.12/prepos/files/Artemis/Public/Recalls/2003/V/RCFK-03V474-8795.pdf
http://152.122.48.12/prepos/files/Artemis/Public/Recalls/2003/V/RCDNN-03V474-6676.pdf
and this next link was part of a previous post in this forum about the Campaign, people were skeptical of it, but its hard to deny with the above link showing the actual letter of acknowledgement from Volvo.
http://www-odi.nhtsa.dot.gov/cars/problems/recalls/recallsearch.cfm

Good Luck!
Dane