Engine knock or is it the infamous au rattle?

[TeamSAXON]

A friend today pointed out to me that my engine was knocking, I don't often hear it and when I have actually heard it, it's been a hot day with the ac going.

Car is an xr6, the engine has 367xxx on it.

First off, what is the sound difference between a knocking engine and a rattling engine or are they the same? It is pretty damn quiet when just idling but if I rev it up into 2500rpm range it starts knocking or rattling.

It stops or gets almost indiscernible when the engine goes back down into idle. You know it's there, it's just not loud at all.

Now this time of year I can't get my mechanic out for a while.. Would this be something I need to worry about or a major indicator of a serious problem? I've read on this forum the rattle is common?