Any hints at this noise?
If the noise appears when you create more torque it's almost certainly bottom bracket or crank related. You can get creaks in the rear wheel but they tend to manifest in lower gears because that's when the torque on the rear hub is highest.

Cannondales are fairly notorious for creaking in the bottom bracket. The problem is that the bearing cups are slightly oversize which allows the outer bearing races to move. Bottom bracket bearing replacement would not necessarily fix the problem as the cups remain oversize.

Chainring bolts can also creak so remove, grease and re-torque those to rule that out as the cause before purchasing more expensive parts.