FPGA drive current effect on power draw in Cyclone IV

It is ringing, not noise, sorry. And 24 mA wasn't a design choice - it's the default setting on the Cyclone IV. I don't think we realized the current was settable until we started looking for the cause (the problem didn't show up until after we did a board turn, but now that we know what to look for, we have found it on a couple prototypes). (In the "not helping" category, the engineer who did all the heavy lifting on this product for the last fifteen-plus years retired recently. We did ask him, and he thinks it's the simultaneous signal switching as FvM suggested.) My theory is that the bypass caps are right on the margin.