I have had the same issue. What began as movement in the ISOspeed seatpost eventually became a frame crack on 2 different SLRs. I am on my 3rd replacement frame in as many years.
In every case where the frame cracked, it was an audible, very loud crack while I was riding. The most recent crack is on the underside of the top tube in a spot that is not visible without removing the ISOspeed entirely.
The previous crack was in the ISOspeed bolt hole. The bolt had backed out and caused a big piece of carbon to “chip” off. Prior to the first crack, I was plagued with creaking and the bolt would not stay tight under any circumstances.
I will add that Trek has been very responsive to every warranty claim. Everytime Trek replaces the frame, I request compensation in product.