Cracks in front or rear cats, where to search ?

[spongebob]

Thanks for the info Jake D, guess that this is the best way to do it.
I have tried to locate the leak with my fingers but it gets warm in less than a minute so you have to be fast. The stethoscope will be a better solution.
I did a search for stethoscope and ended up in an article where doctors used Iphones as stethoscopes but that app is probably not invented for cars (yet) so I will have to find a regular one.The stethoscope will also give me some possibilities to listen to the IMS until I have it done