Odd throttle sound at certain positions...

[98XJSport]

I have a 98 XJ 4.0 with rustys airtube, tb spacer, and a flowmaster catback. My problem is when i let off the throttle almost but not quite all the way it makes kind of a wierd gurgle, vibrating sound somewhere in the exhaust im assuming as it sounds behind/below me. It does the same if im coasting with no throttle and then just give it slightly. I mean slightly as in coasting through a 15mph school zone, barely enough to keep speed. This seems to have happened only since i heard a rattle (broken piece in the cat) and replaced the cat. Any thoughts? I didnt post any lift specs doubting that they have any impact....

Thanks

[XJ_ranger]

rustys airtube, tb spacer

thoes are more than likely the noise causers....

i get a whisle at certian throttle positions with this one:
http://www.opiebennett.com/customfipk.htm

how long have you had:
rustys airtube, tb spacer

???

[98XJSport]

Ive had those for many months, they are not the cause of the problem. Yes they do cause noise, in the engine compartment. As I said before, the noise is behind/below me. And only in the last few weeks. The only changes ive made in that time is the addition of a BB on top of a 3", and a new cat. I also noticed today that the sensor on the cat, not sure what its called, was unplugged, causing my check engine light to come on. Not sure if this has any relation to my problem.