Mellotronists

Hi y'all,
Anyone out there able to help with digital recording (or rather 
playback interferance) problems? I get popping/crackling when 
playing back .wav files in Cubase. I've tried adjusting buffer sizes 
and cache schemes etc to no effect. My HD speed is 7500 but the 
popping does seem to coincide with the HD led. Any ideas before I 
tear out my hair?
John