Rattle noise 97 Blazer

This Blazer is in really good condition, had it for a year now. Lately or for the last month or so I started hearing a vibration buzz/rattle sound almost like a bolt or bracket loose underneath somewhere. You will hear it mostly when idling in park but when you put it in drive and are moving you wont hear it. Really sticks out when in a garage. It runs good, just tuned it, transmission feels good, it really does sound like something loose underneath, problem is of course what? and how to nail it down. Any idea? thanks....

When mine made that noise, it was because the bolts that hold the skid plate under the tranny and tranfer case had come loose.

so what does this skid plate look like exaclty? thanks

check the heat shield on your catalytic converter sometimes it will come loose and cause a rattle.
also the honycomb material inside the cat will break
up over time and make a rattle noise its rare but it
happens.

Get out of the truck and crawl underneath and look for the rattle.