Here's my subjective, amateur comparison of four single platter drives:
2.5" 40gb 4200 Fujitsu MHV2040AT Thailand mfg 4/05
2.5" 40gb 5400 Hitachi 5K80 HTS548040M9AT00
2.5" 40gb 5400 Toshiba MK4026GAX
3.5" 80gb 7200 Seagate 7200.7
Fujitsu in Dell 9300: Idle is extremely quiet, inaudible to me from normal distance when using notebook even with quiet background. Putting ear to side of notebook, can hear high pitched noise at low volume. Seek/read noise is audible from distance, but very quiet and not of annoying quality for me.
Fujitsu in enclosure with top off: Idle is not as quiet as I expected, and has an annoyingly high pitch component, depending on angle of drive compared to ears. Seek/read noise is fairly quiet.
Hitachi in IBM Thinkpad: Idle much louder than above, but to me, quality of noise is not so bad. Whoosh sound. Seek/read is perhaps same volume as above.
Toshiba in Dell 9300: Idle is a touch louder than Hitachi above, but of much worse quality to me, high pitched and disturbing. Seek/read is much louder than above drives, but not of a bad character to me.
Toshiba in enclosure with top off: Idle is similar to Fujitsu in open enclosure but probably louder. (Oddly, it wasn't night and day difference to me. Perhaps too much time passed.) Seek/read louder than Fujitsu in enclosure.
Seagate in closed encosure: much louder idle and seek/read noise than above drives. Both idle and seek/read are of annoying quality to me.
Conclusion: For me, the Fujitsu was well worth the $66 total price, to replace the Toshiba in a notebook.
mdubrow, the 80gb version of that drive was just reviewed on this site, and not found to be so quiet. I wonder if the difference is the number of platters. In any case, glad yours is quiet.