Odd noise while stopping

I have a '98 Intrigue (100k miles, Vin K 3.8L) that makes a quieter version of the yatzee shaker sound when slowly braking to a stop while the tranny is downshifting from 2nd to 1st. It seems to be less pronounced with the traction control off, and seems not to be there at all when braking hard. Turning right or left during braking makes no difference. It has been doing this for the past few months. My first thoughts were that it was related to the ABS or traction system since the tranny is shifting well, but no lights have come on in the dash. Any thoughts would be greatly appreciated.

Does the brake pedal move of its own accord at all when this happens? It would have a feeling of fighting your foot.

Thanks for your reply. No, the brake pedal feels like it has since day one. It doesn't pulsate like the ABS is trying to kick in. The tranny fluid level is OK but is due to be changed in the next 2k miles (I may do it sooner to troubleshoot this noise). I'll do some more experiments this week like shifting manually with and without the traction control engaged. I just finished doing the upper/lower intake manifold job last week and am trying to get this car in tip-top shape to last a while.