Exhaust leak in manifold/front cat

[spongebob]

I have since I bought the car an irritating noise from engine compartment.
More or less the same sound as a ticking valve. But from several people I have heard that it is an exhaust leak and if I remember correct from the PPI a long time ago (2006) the mechanic told me that the cats had to be welded. Since I have cat bypass pipes, only the front ones are left.
It is very difficult for me to find these cracks as the noise disappears after 1-2 minutes and they becomes hot.....
As removal of the cats probably will damage some of the bolts so it would be nice to find the cracks to see if possible to weld in place.
Has anyone else experienced the same noise and found some kind of cracks in the front cats?