Weird engine sound at low RPMs

So for several weeks I've been noticing my engine sounding, well, just odd. I couldn't find anything in the stickies or previous posts that sound anything like what I've got going on, so here it goes.

When idling and when driving in low gear at lower RPMs (around 1000-1800 or so) the engine makes this awful, growling, roaring sound and just sounds terrible, and doesn't drive as smooth as it should. Once I get up in to higher gear (driving anywhere above 2000 RPMs) it sounds and drives just as smooth as ever. I haven't taken it to a mechanic for this because I honestly have no idea what to tell them, I have no clue what is, and most mechanics around me have virtually no experience working on infinities. (I live in GA, where everyone drives Fords or GMs, and that's pretty much it).
I recently had a problem with the throttle being clogged, it was revving up on it's own while I was sitting with the break on, and got that fixed, and just got the oil changed about a week ago. All the other fluids are within normal levels, and it's still doing it, and honestly getting worse. I've also just today noticed a slight rattling noise when driving in low gear as well. I know it's a long shot with not much to go on, but I have no idea where else to go to for help. Any ideas?