I have a 2000 S40 with 51,000 + miles, and I have NEVER had the noise you describe!! Is your trans. fluid nice and rosey pink? I had to have my tranny flushed at 33,000 miles due to fluid discloration from the winter's driving in using my DSA to its fullest measures!
If you have to flush the tranny, use an independent dealer-service who uses the BG- PF5 power flush and fluid exchange service procedure, whereby ALL of the old tranny fluid is flushed out of the tranny with the dissolved deposits due to a pre-treatment of a cleaning chemical. A conditioner is installed along with fresh fluid to extend the fluid life, and conditon the tranny seals, so as to prevent leaks and expensive repairs. The manufacturer of this equipment recommends the procedure every 30,000 miles, vs Volvo's recommendation ONLY WHEN THE FLUID CHANGES COLOR, OR AT 50,000 MILES WHEN ONE PULLS A TRAILER.