[userbinator]

The manufacturer worked around the problem by adding a custom filter in the audio pipeline that detected and removed the offending frequencies during audio playback.

Too bad the manufacturer wasn't named; I quckly looked through a few laptop schematics from that era and didn't find anything that stood out as being a notch filter.

[pjc50]

You wouldn't see it on the schematics, it's in the driver.

[cenamus]

I imagine that it would also be quite difficult to realize such a narrow filter in hardware, 90 Hz is already quite low.