Grinding noise in reverse

[NickNickNick]

So, after replacing the engine and clutch in my 2006 Yaris, it's been driving just swimmingly for the past 6 weeks.

But, today, I noticed a strange noise (a lower-sounding grinding) when I put the car in reverse. Obviously, it's a manual transmission. Like I said, I just had the clutch replaced about 6-7 weeks ago. The grinding doesn't shake the whole car or anything, it's just a brief low rumble that can be heard about 60% of the time when I start into reverse (ie, when I begin to idle the engine higher and as I start to let the clutch out). It almost sounds the same as when you get a little packed snow or ice into the undercarriage. I have included a link to a video where the sound can be heard. It's the low grinding just when I start to move backwards.

It also doesn't seem to happen all the time - if I let the clutch out really slowly, it makes no sound. If I just give it a little gas and shift to neutral right away, it makes no sound. Only seems to make a sound when I let the clutch out completely - and even then not all the time.

Anyone know what this new problem could be?

Thanks.

Here's the video:

http://www.youtube.com/watch?v=xqhlglTIZoU