sound cards that actually make sounds

i noticed something weird yesterday: my four-year-old delta 44 generates a high-pitched whine, with a frequency and volume depending on the samplerate it's running at.  i don't mean that it's adding noise to the outputs; the actual physical card itself is producing it.

it's most noticeable at 96khz and 48khz, where it's an f# (around either 2960 or 5920hz, possibly making it a subharmonic of the clock frequency).  the 96khz whine is by far the loudest.  it's also there at 44.1khz, at a lower volume and a higher frequency (too high for me to easily determine pitch).  you can hear it the same way you can hear whether a crt is turned on.  also, it goes away while i'm changing the gain on any of the adcs or dacs.

it's not a huge problem; it doesn't seem to be leaking into the i/o at all, and with the case on my computer, it's only even remotely audible at 96khz.  but i'd be interested to know if any other delta series owners have noticed this.  is it just mine?  is it just old?