Funny Exhaust Note

[xxxBlakexxx]

I need the experts for this one:

I have noticed that the last few times I have taken my car out, a funny sound is comming from the exhaust or the rear-end, but it may even be originating from under the hood.

During WOT, as the rpms pass 4,000, is when I hear the sound. Almost sounds like a rumble. As the rpms go higher, it goes away. It is not real loud, just different and not the way it used to sound.

I have made NO changes to the exhaust and I feel no loss of power. I suspect perhaps something loose on the exhaust system, but it is not a metal on metal sound.

Here's another thing, when sitting in the driveway with no load on the engine, the noise can not be duplicated. But, as the rpms drop down, I can hear a slight crackle...almost like straight pipes but not nearly as noticeable. I don't know if this is related to the rumble or just me hearing something that was always there before...just never noticed.

I suppose too that the rumble described above could also be comming from the rear-end and not the exhaust...perhaps even the limited slip kicking in.

Any ideas?