'01 Cavalier growls

I have a manual '01 Cavalier and it is making a weird noise. When at low rpm's for a particular gear there is a "growl" emitted. I have heard trucks make a similar sound before but never a car. The car shifts and drives with no problems. It seems to be in pretty good condition. This is not the same harsh noise as gears grinding and the clutch works great as far as I can tell. I'm just wondering what is causing this noise and if it is normal. For example, if I shift into second a little too soon, it will growl until rpm's are a bit higher then it stops. It is not just after shifting either, if I slow back down to low rpm's it happens as well. When this happens, the rpm's are not so low that the car might stall and it does not shake as if it is about to stall. If anybody knows what this is or might be I'd appreciate the feedback. Thanks.

Mine kind of does the same thing around 1500 rpm's when shifting, or sometimes when shifting from 1 to 2nd uphill. It's not loud or anything, so I suppose it is normal. But if it is loud for you, then I don't know what it is. BTW I have a 2.4L engine.