New headers causing buzzing nose ?

I recentlyl purchased some new DC SPORT 4-2-1 headers from a friend who used them on his SI for a year. I got them at good deal so I'm happy with them. My concern however is that in order to install the ehaders on my 98 DX I had to remove the cat. My car went from a 17.2 stock to a 16.6 with intake and exahust. Since installing my headers I've seemed to drop back into the 16.9-17.0 range...reaaaaaaaalllly slow!!!!! But what concerns me more is that when I hit 3000 rpm my exhaust makes this buzzing noise and it's pretty loud and it conituens until i hit about 3400 or 3500...in order to install the headers I had to use one 02 senosr and the other is tied to the engine bay. My check engine light oems on every now and then and I"m just curious as to what the noise is and how to fix it. Should I get an aftermarket cat or what ? Please help!!!!!