I have a 2002 Rodeo 3.2l v6 and had the same problem very recently. The noise slowly got worse, always right at 2500 rpm and I couldn't figure it out until it got bad enough, the flex pipe had rusted thru and was leaking pretty bad. I looked around online and found no simple fix, the only answer I found was to replace the whole Y pipe so I took it to the shop and they welded on a new flex pipe for $200.