Recently I got myself couple of cool big CoolMaster computer cases. Anodised aluminium, plexiglas side panel and convenient block of USB-Sound-ESATA connectors... Being very pragmatic about computers I anyhow moved my current machinery into the new temples. The main goal of this move was to make them QUIET. Indeed, the monster cases had huge (read silent) fans, plenty of space for proper heat-sinking and arranging the air-flow. I even decided to glue some dumping material on that enormous back-cover. The day passed, everything has settled well and the machines ran much, much quieter than it was before. Till I sat to work a bit at night. There was that endless "RRR" sound. Not very loud, but it was something new in comparison to the old clunky sharp-edged iron boxes. Perhaps this "new" sound was always there and simply masked by the old little fans hissing aloud. Who knows, but that night it was unbearable. I soon realised the "RRR" was about 120Hz. No need to measure the frequency - you guess it quickly: that was the beat of 7200RPM hard drives. Most probably the big case made much better low frequency resonant chamber for them.
I did a quick marketing research,