Rattle/Rumble/Knock from exhaust manifold

I've got an 89 Cherokee 4.0L with 108k on it and lately I've been hearing this rumble/rattle that I initially thought was a bad electric fan but it seems to be coming from the exhaust manifold area. The sound begins when I start the engine and does not ever go away. I just recently bought the car and I'm not sure when the last tune-up was done but I was thinking of doing one anyway but I'd like to know what might be causing this noise so I can take care of that along with the tune-up. Thanks in advance for any info.