[metroschultz]

you need two people,
one to run the engine - one to listen for the source of the noise.
I agree from what info we have it sounds like an exhaust rattle.
Maybe a broken catalytic converter,
sometimes the strata inside break free of the casing and rattle like the chains of death.
sometimes the strata crack and separate and rattle not quite as bad.
but, back on subject,
someone with a good ear has to find the source.
Then you can decide how to fix.
Weld the broken heat shield or replace the cat?
easy enough either way.
Given that you have a Hyundai Accent, and I've worked on more than a few, I would say from the info "It happens at all speeds and is pretty loud already at idle" the front cat has broken free inside.
Pretty common on those cars.
Not real cheap to fix but definitely easier than welding. all you need are 12mm sockets and 14mm wrench,& deep socket.
Have a Friend stand in front of the car with it running and tell you where the noise is coming from before you run out and buy anything.
Hope all goes well.
S.