Wierd noise in 164LS exhaust

[Alfisto@domain.elided]

Yesterday while going slowly around a corner in my LS/24V, I hit a pothole, 
of 
which there are many on our local hillside streets. The car nose dived and 
there was a loud clunk 
as my exhaust manifold hit a personhole cover. Besides puttting a large dent 
in th e curve of the front header there immediately emanated from the header 
a loud ticking noise at idle 
which refuses to go away. Does this signify that I have cracked the header, 
the downpipe or something else. If a header, can it be welded or otherwise 
repaired, and how easy/hard to fix or replace ?  I checked it and could see 
no signs of a crack nor trace any exhaust leak. It sounds as if the tick 
might come from a bad valve in #3 cylinder. Could by any chance the dent ( in 
the curve) cause an increase in back pressure which might go back and have an 
effect on an exhaust valve when at idle, but go away under load ?
Graham Arlen

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