2002 Volvo S80

I had a bad power steering groan (not a whine or squeal). Dealer and independent both said it is the pump. Both said the rack is fine. The dealer thought I might have a resevoir leak as the fluid was a little low. Although both said it was not necessary if I monitored the fluid level and could live with teh groan, I replaced the pump and the reservoir. Once bled, the groan was gone for 15 to 30 minutes of driving. It was then reduced but back. It is silent after sitting overnight until I drive 15 to 30 minutes and then it starts. Quiietly at first then it gets worse. This is on going for about 3 weeks. The steering is otherwise perfect, just groans (a low gerrr sort of sound but no real shake of sensation in the steering. I see no signs of a leak (cardboard under the car each night and I keep a watch on the fluid.

Anyone ever had this off and on groaning that goes away after 5 plus hours on non-use, but returns after driving a while?