On my 1995 Toyota Corolla I noticed a rattling noise coming from under the hood and thought something had happened to the engine or at least something came loose. After checking everything out and finding nothing I let the car idle for a while to see if the noise came back. It did not. But I knew for sure that I had heard a rattling sound coming from the area of the hood and I needed to find it just in case it turned out to be something serious.
I decided to make the idle really rough by removing a vacuum line and cause the engine to sputter. It isn't a good idea to do that for long, but I needed that type of vibration to help find where the noise was coming from. Sure enough, the rattle returned and it turned out to be nothing serious at all.
On the hood of the Toyota Corolla, as in most hoods, there is a frame underneath the finish sheet metal, the places where the frame and the sheet metal meet is caulked at the factory to prevent rattles and the top sheet metal on the hood from flexing.
What had happened is the caulk started breaking off and was rattling around inside of the channels of the underlying hood frame. I uses compressed air to blow most of the broken pieces to one of the drain holes so I could remove it. Once the chunks of broken-off caulk was removed the rattle stopped. The caulk was not damaged even though pieces of it were rattling around. The caulk that held the frame to the hood to the finished sheet metal skin was still holding strong so there wasn't really any need to do anything but remove the broken off pieces of caulk.
If your Corolla hood rattles and you notice that the adhesive has come loose between the hood frame and the sheet metal skin just fold up a piece of paper and wedge it in between the hood and hood frame to create an opening then apply an epoxy that remains flexible. Once the epoxy dries remove the paper.
