something you all might want to check over

Motordyne

Can you think of anything that may be related to, or caused by, the bolt tightening? If the sound started upon bolt tightening, something else had to happen because a tightening of bolts (by itself) won't do that. What else could have been part of the bolt tightening process?

Cold starts can sound different.
Could anything downstream on the exhaust have been affected in any way?

Also see if you can isolate the location of the noise. That will say a lot about what the solution is.