The original Big Dog noise problem came from using an off-the-shelf fixed speed engine/hydraulic pump combo. There was a subcontract for the LS3 to replace that with a quieter custom-designed Diesel-powered unit. (The US Army is all-Diesel; they don't want gasoline tankers on the battlefield.) I'm not sure how that came out. The original Atlas was a Big Dog derivative, and used many of the same hydraulic components.
The Schaft robot (also now owned by Google) is all-electric. That's probably the future direction.
The Schaft robot (also now owned by Google) is all-electric. That's probably the future direction.