Engine moans!!

After a relatively hard run, such as a steep highway hill climb, and usually on warm days, I get a "moaning" engine noise at low rpm's. It is totally rpm-dependent and present even when not moving so is not a driveline issue. It changes sound as I turn the steering, though I think this is simply the PS pump dragging the rpm's down slightly. The moans are not consistently there... only after pulling hard. Sound goes away after a minute or so of idling. 2001 Disco V-8, 40,000 miles, no codes or lights. Does not affect performance. Any ideas?

Thanks.

Chuck