I have a metallic whistling sound coming from the back end, passenger side. It happens only when I floor it at low rpms, even in N. At higher rpms (3k+), revving the engine doesn't produce the sound.

It sounds like if blew really hard into an empty tin can.

Makes me think fuel pump but I don't know. I took off the rear seat cushion and it sounds pretty much the same... I hear it working, but that's another sound.

Any ideas? Exhaust?

Sorry I cannot help you but I have the same problem. It stops once you fully engage the throttle. I changed my exhaust and it solved nothing. Thought it may be the tires but it never stopped after I changed them. Hope this at least clears two poss. Good luck!

could it be the brake linings for the parking brake are worn?

Most likely to be exhaust - a small leak, or a partially blocked / collapsed baffle in the rear muffler might cause a whistle. Perhaps at higher revs you just can't hear it over the increased engine noise? Oops, just realised your passenger side is RHS - so do you have a twin exhaust system - if not, maybe it is the fuel pump ....