Me and a buddy installed headers on his 94 5.0 mustang. We got the header bolts as tight as possible but there is still a tinging noise. Is that common with headers. Went to a car show last weekend and asked 2 other people with headers and they said that they all had that noise.
I'd inspect the collector gaskets. Until the exhaust system settles it'll keep tearing non-copper gaskets and sound bad. While you're checking them I'd replace the collector bolts with stainless ones. It's worth the five bucks to go buy stainless.
If it's not that, I'd try running higher octane gas. You might have spark knock that you couldn't hear before because of the manifolds.
Headers are noisier than the stock manifolds. They are thin tubing and can't absorb sounds like the cast manifolds. Any noise will also be amplified in the tubing and sound like the sounds you are probably hearing.