Weird rattle noise/cold start choke @low gear

my '07 sentra has just under 52K and has had all regular maintenance. for the last 6 months or so it’s made a faint rattle, it sounds like from the rear passenger side. no one can find it. and now in the last 2-3 weeks it has really rough cold starts and almost chokes/stalls at lower gears. once it’s warmed up it runs fine, although the rattle is louder when it idles. i can’t get it to reproduce the stalls for my mechanic, but it’s definitely there!

The rattle could be from inside the muffler. Which side is it on?

For the cold stall have fuel pressure tested when cold.