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I've posted a ton of stuff on "the buzz" over the years. You might search for that. Meanwhile, I often find that the buzz is not coming from where it sounds like it is. If you have determined it is the exhaust manifold heatshields and not the shields on the downpipe or the headpipe under the car before the cat, definitely check the bolts that hold the shields to the manifold. They can be tight and not be tightening the sheet metal. As long as you can get them out, try adding a stainless steel fender washer under them, on the manifold boss, as well as under the head of the bolt. This provides rigid support for the sheet metal and clamps it well. Make certain the bolt hasn't sheared in the manifold leaving only a small portion threaded into the boss. Next, they may be tight, but the leading or trailing edges may be bent such that they are close enough to touch when the vibration reaches the right level. My best success at finding the buzz involves my son. He has gotten quite good at holding the throttle at exactly the buzz point while I pry around all the heatshields with a long screwdriver until I find what stops the noise. Most common culprit for me has been the bellhousing to headpipe bracket and assorted hardware. There is a bridge across the headpipe, the intermediate bracket and the bellhousing bracket. The bridge on the headpipe has a captive nut under it that loves to fall into the crotch of the pipes and hide.
Good luck!
DS
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