Clicking while I coast
 

First, let me just say that I'm pretty illiterate when it comes to vehicles, so forgive me if my terms are a little off...

I've been hearing a clicking/rattling sound when I coast in my '06 RX-8. When I have my foot on the gas, the clicking goes away. At high speeds, the clicking becomes more of a whine (just really fast clicking). In other words, I believe the clicking is directly relative to the wheel rotation, but being as vehicle illiterate as I am, I don't know that for sure. It just seems likely...

With that being said, I took the car to a mechanic friend of mine, and he replaced the rear differential oil (or some such thing...) and it was incredibly black, and had a small piece of metal in it. Obviously that's a bad thing, and he suggested I have the transmission replaced, because he thought the clutch is what was causing the clicking sound...however, I have the sound whether I have the clutch pushed in or not, and I have absolutely no problems shifting in and out of gears.

I don't want to pay someone to "fix" my transmission if that's not the problem. I believe it may just be the gears that turn the rear wheels, but I don't know enough about cars to say that for sure. I have no idea what the nuts and bolts look like under there, or how any of it works. I just know what I hear, and when it happens.

Any help/ideas would be greatly appreciated!