Nathan,
If you do a search on jgveill you will find a post in noise control on the 6th page that has spectrums (FFTs). Surprisingly (to me), there are no visible lines or spikes and the levels below 350 HZ are increasingly large. Also, the spectrum is very wide with significant high frequency content. So I doubt anything like simple resonators would be very effective. People have had good results with the closet and lengths of insulated duct.
bababrown