Is there any way to reduce the noise and vibration in a C-130 Hercules?
Active vibration control systems exist for some aircraft, though I doubt an ex-military type would have anything off the shelf. There is a system from Lord (the engine mount people) that is intended to counter the vibration from a helicopter main rotor to improve passenger comfort, among other things. This is their claimed reduction:



AVCS Product Brochure

They focus on helicopters but sell components as well, and may be able to customize something that could work for you.