my 93 civic is a DX...good car but after a minute or so of the AC being turned on, the underside of the car rattles real loud - i jacked up the car and ran it for a while to see what it was. I thought it was coming from the back of the car (that's what is sounds like when I drive it) but it is actually coming from under the front end of the car -
I dont know much about cars really so I am hoping someone can help me out here - From the back end of the car you have the muffler, then the catalytic converter, then that same metal tube runs all the way to the front end, where it is joined by a header....that is the thing that is vibrating. Not just a little but a LOT..so much that when I do run the AC, the people next to me on the freeway cannot help but stare at my little blue car.

I wouldnt be worried about it but I have someone interested in buying this car and I want to make positively sure that I know exactly what that sound is before the test drive day rolls around...does anyone have any advice? Thanks -

Sounds like it might be a heat shield. The a/c changes the idle of the car, which could be why you only notice it when the ac is on. Try to get a screwdriver, and put pressure on the suspected part while it is rattling. Use common sense, dont poke around the belts while they are moving, but the manifolds are fine.

If it is a heat sheild, just remove it, they aren't really needed, unless you plan on parking in tall grass.