Both of our 2000 Bonneville'* make a loud whistling noise from deep inside the dash when the climate control is in the vent position. (or bi level.) It comes from deep inside the dash and changes with fan speed. It is very annoying. anyone else have this problem and if you do/did what is making this annoying noise?!

When was the last time you changed the interior air filter? Sounds like its getting plugged?

the SSEi has a clean newer (1 month old) filter, the SLE is running no filter right now because it was really dirty and plugged up.

The filter can be a pain to fit and a filter fin' maybe bent out of shape, like a blade of grass whistle you would play with as a kid?
Try removing the filter to see if that helps or at least changes the pitch?
I know my system would whistle when the filter started to go and was really loud when the I removed it awaiting a replacement (ebay special), its quiet now with a new filter.