Good info and solution. Slightly adjusting the bracket position is possible for proper mounting alignment.I had same creaking issue when peddling hard. Replaced all 14 suspension bearings - still creaked. Found reviews like this on motor mount creaks so tried that next. I found that all of the bolts were still tight and properly torqued up. I did notice, however that the frame was rubbing against the motor housing (see pic). Looks like the original build had the motor a bit too far to the back causing this rub (and creak) between the housing and the frame under load. Since there is a bit of play on the bracket holes , I shifted the front mounts with these brackets adjusting the motor a tiny bit further forward. When re-mounted, the back plate screws are accessible so after bolting the front to the frame, I simply adjusted the back plates to line up with the frame holes. Note - in an abundance of caution, I also added some anti-seize between all metal to metal surfaces. No more creaks. So even if your bolts are still tight, look for this problem.
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Bracket bolts becoming loose or shearing off seems to be a problem on some older Trek bikes. On a recent Trek bike I discovered a very small crack around the motor mount insert. This allowed small movement of the motor which eventually led to the bracket bolts shearing off. The crack is barely visible but there is some very slight flexing which leads to motor movement.
IMO this led to the shearing of the bracket bolts. The frame is at fault. We will see if Trek will honour their Lifetime warranty on frames. They could easily explain this away as “operator abuse “ or “extreme abuse “. This could cost him a frame and motor.
I know the owner is not an aggressive rider. No extreme jumps , some drops, …average riding imo. He has less than a 1/4 of the mileage that I have (exact same bike and year , Rail 9.7).

