Fine408C5R

It just started doing this today,everytime I crank the car I hear an odd rattling noise coming from around the bottom of the motor.I was guessing maybe an idler pulley or something around that area causing the racket it has great oil pressure so I'm ruling out the scary stuff like rod bearings or main caps.After driving the car from work as I got closer to the house I came to a stop sign and the car bucked down to 300-250 rpms for a split second then went back idling fine,then I feel something vibrate when I got home I let it run for a bit and it done the same thing several times I checked under the car for leaks it don't use any oil whatsoever this is making me nervous because the car has never bucked at idle and surged down in rpm like that then having a vibrating feeling coming from somewhere...I just hope it ain't nothing serious, it drives fine with no hesitation and shifts smooth. I really need some help on this please thanks!