Sampling problems

Hope you guys can help but my MPC 4k will load samples from usb perfectly but when I sample them they are crackling what could be the problem and how could I put a sd card reader in it instead of a cd rom

are they crackling in the headphone out or your main out ?
sure you didn't overload when input?
are there filters active on the PROGRAM?
the gain staging on the 4k is particularity 'nasty', i mean you have to watch out it doesn't overload on all levels (multi, program, EFX...)
there isn't an FX active when sampling ?
what about FX on the output/Multi ?

It can be related to your RAM memory. Try reseating/remove it.