Possibly try some 10w-40...since this summer has been a VERY hot one...oil breakdown could be premature under hotter instances.
If it quiets down then that is the easiest diagnosis ever. The 3800 has a noisey, clattery, valvetrain...it's common really. Which is why I always put in 5 quarts every oil change and by the time I go to change it...there is the recommended 4.5 in there...which is about normal wear and breakdown. That quieted down the silver car by A LOT.
I've never heard a loud injector on a 3800, just a vehicle with DI, not your everyday MPFI, or FI. Also...with the breather off the motor will be a little louder because it isn't on there to muffle the internal noises.
Have you yanked the plugs to check their condition as well? Just to rule out WCS...ya know.
