Steering Chatter

On a 2001 Outback Wagon, I have a steering chatter at low engine rpm only. Started at about 55k miles, when the serpentine belt was replaced by a dealer. At the same time, the p/s pump began to whine. I had the pump replaced, and the whine left, but the chatter remains. Vehicle has 61k miles. The dealer, who first said it was the p/s pump (and replaced it to the tune of $500+) now says it's the steering rack, to the tune of $1000. I am suspicious of all this, but can't find anyone with experience on it. I am told that it's rare for a rack to go out. Anyone know anything about this?