When I installed my up and dp, I had a similar problem. I had a bad gasket and also, one of hte bolts, I had a hard time getting to on the dp. This caused a small leak. Just make sure you tighten all the bolts evenly. Don't tighten one all the way and move to the next one. Tighten a little on one, then move across to the next bolt and tighten a little and so forth. This helps keep the both the gasket and the collector from bending/warping. I know it is a pain in the rear, though. The up was a huge pain in the #$%!! I don't have any leaks now (knock on wood).

 

Kind of a shot in the dark, but did you double check the sensor that goes into the up? It almost sounds like something is loose and is rattling and at that rpm's it just may be rattling it the right way so that it resonates through the metal piping in that annoying way. I'm sure you have already done this also, but think back to every single bolt you took off to make sure there isn't something you have forgotten. It is almost sounding like something is loose. Did you put the heath shields back on? If so, check to where the turbo ones go over the dp because you may have to cut them a little to make them fit over the piping correctly. Just throwing out ideas. I know how frustrating it can be when you buy something, install it, and then it does something like this. I was so freaking pissed off when I first put mine on I started throwing things in my garage.