Play around with the levels on the back of the monitors and see if you notice a difference, after that get a pair of headphones and plug them and see if it's still there, if you hear it with the headphones (or other source) its most likely the mpc. If you don't hear it its probably the monitors. The Rockits are great monitors and i love mine to death but they arent the top of the line in reference speakers. Ohyeah and make sure you dont have the volume on the monitors cracked all the way up, I run mine through a mixer but I have the volume set no more than half way up, but usually less. Also like the other guy said, what kind of sound is it making?...Also with the samples you gotta make sure that they are the right bit rate and hertz, which is prolly why youre hearing it on some and not others, you wanna make sure its at 16bit and 44.1hz, I use db power amp to convert all my samples and sounds. But thats more for if you're loading the samples on a disk from a computer, if you're sampling directly into the mpc from an audio source you shouldnt have a problem, in which case if it is it could be you're ins or outs...If all else fails look into your ram