I've had my GT for 3 months and just over 2800 miles now, and it seems to have developed a peculiar metallic-sounding rattle emanating from under and (I think, can't quite exactly pin down from where) in front of the car, as if something was loose, and it seems to occur only in 4th gear (this rattle doesn't occur when I shift into 3rd or 5th), at speeds around 45-50mph and below, and under any kind of acceleration; the rattle does not occur while at a steady speed or while decelerating. Has anyone here experienced anything similar?

 

That sounds a little like ignition knock. Often caused by carbon build up on the pistons or poor quality low octane gas.
If you put any kind of custom tune on the car and then run regular gas, you'll probably get the knocking or rattling sound because it advances the timing. If that's what it is, it can cause serious damage to the motor if it continues. If you aren't sure if that's the case, try running some premium gas for a few tanks.

 

My car is still bone stock, but I'll try running a few tankfuls of 93 to see if does anything about the rattles...

 

Probably not related to your problem but in my 05 at around 6K miles I started to get that grinding/growling sound listed in the applicable TSB for the 05 manual. I would call it more of a sound like a fan motor with a bad bearing. It would come and go, it first started in third but then occurred in 4th. I think I remember a post from a guy with a 06 having a similar problem

 

It shouldn't be pinging if you haven't changed the timing unless you got some really bad gas.
I'd take it to the dealership for a checkup if a couple of tanks of good gas doesn't stop it.