Recently doing a condition inspection on CC11-160 we found wear on the #4 stack from the heater shroud. These shrouds are fit with bent tynes acting as absorbers for vibration. With time and continued vibration, the shroud can loosen and start to wear holes in your stacks like this one. This example is at 550TT. Please include this as part of your inspection process and tighten as necessary. Thank you Mark