A friend's GLOCK (compact model)'s slide has a rattle in it when the gun is uncocked.  I have field stripped the gun and verified it is in the slide.  When I cock the stricker, when the upper is off of the frame, the rattling stops.  

The gun was "accidently" assembled minus the barrel and recoil spring (for storage) and this was first noticed afterwards.  Is this anything to worry about?

Thank you ahead of time.

CSP

When the gun is together and uncocked the striker should rattle in the slide when the gun is shaken.  If the striker deosn't rattle, or is less rattlely than usual,  then you need to detail strip the slide and clean the striker and its channel dry.

So with the gun cocked, no other rattles right.  But uncocked it rattles when shaken forward and backwards (like recoil).   Thats perfecly normal.

Right, it's only when the pistol is uncocked that the noise is present.  When it's cocked there is no noise.

CSP

Its supposed to do that, if i remember correctly its the firing pin safety. No worries....

Cool, then no worries.  

Pretty sure it is the firing pin/ striker.  The safety would rattle even if the gun was cocked, I think.  And the FPS spring should hold the safety still.  

When the gun is cocked, the trigger assembly holds the striker half cocked  against the slightly compressed striker spring which is why it no longer rattles.

I was told if it stops rattleing then the firing pin/ striker assembly is too dirty and needs cleaned.  If it isn't clean it can jam or slam fire.

Nothing wrong with it, OP.  It's the Firing Pin (and yes, in Glocks it's called a Firing Pin.)

It's supposed to "rattle" like that.  If it doesn't, the Firing Pin Channel needs to be cleaned out.  Never put lube in there, by the way.  That area needs to stay dry.