If I remember right, that shield is not metal but some sort of heat resistant material and if so, can't be welded. Is the bolt head itself missing or the whole bolt? If the whole bolt is missing, A quick trip to Lowes or Home Depot will solve that problem. If someone before you tried to remove the heat shield, they could have wrung the head off the bolt. Bolts in exhaust manifold heat shields in all sorts of cars are notorious for sticking in the manifold and heads wringing off due to all the heat involved. If there is enough of the bolt protruding, you may get it out with vise grips. If not, you may have to drill the bolt and use an easy out to remove it. as a temporary fix, just cut a small piece of hi temp insulation wrap and stuff it in the heat shield hole. that will stop the rattle until you can get it fixed.I took the car to the mechanic to get the car on the lift.
( We drove the car first so that the sound could be heard. )
Nothing unusual found under the car.
The guy opened the hood and poked around and discovered a bolt missing on a shield.
View attachment 106285
The tech said he has seen this a lot on other cars. The bolt head comes off after a while.
SO, I got an appointment after Thanksgiving to get a spot weld on this. Hopefully that will
take care of the sound/noise.