It has to be under a lack of strain, I reckon; the card's an NVidia GeForce 7800GT. It can trundle along with 2007-vintage games (BioShock, HL2 and Episodes, Burnout: Paradise) with ease, so I find it hard to believe DW can possibly be over-stressing it. Maybe it's just being made to work at a very specific frequency while DW runs. I
did knock a capacitor a bit when I was putting the card in...
ETA: Moreover, the only time it goes away entirely is in the 3D view - the place I'd expect the vid card to be under the greatest strain of all. Unless rendering the control panels is more demanding than I thought.