Oh, so this is while sitting still - in that case, I would lean towards a power steering pump problem. Since it happens hot, when the fluid viscosity is lower, it sounds like not enough fluid is moving through the pump. So I'd start with a level check. A flush and filter replacement might be in order next. If that doesn't cure it, I think you have an internal pump problem, either wear or relief valve if there is one. (Perhaps one of the more experienced xoutpost members can help there). Good luck.

2002 X5 3.0 257,400 miles
2004 325i 114,000 miles