Salut , je te rassure j'en ai aussi mais c'est normal , deja on est a 3200 iso (que j'utilise tres rarement) c'est du banding .
un extrait du review du 40d sur imaging resource

"A minor image-quality strike at high ISO and long exposures: The ISO 1,600 and 3,200 shots here show some evidence of horizontal banding in the dark areas at the lowest light levels. The crop at right shows the extent of this at ISO 3,200 with an 0.6 second exposure. The effect is subtle but visible, particularly when looking at the full image 1:1 onscreen. At ISO 1,600, the effect is less pronounced, but still detectable. Not what we'd call a serious problem, but it could be an issue for some users in critical applications, so we thought it important to call attention to."