Exhaust may be leaking at the exhaust manifold and catalytic converter which is joined together as one complete unit welded together?

Getting raw bad exhaust smell in car when stopped at lights etc, but fine when travelling. A whistling sound when 1st started at cold start up and the sound gets less noticeable when warmed up. Slight hissing sound of air escaping when accelerating. Under the hood I can hear louder exhaust sound with noticeable smell near the manifold area. There is no check engine light on and there is no noticeable lack of power. Looking to find a manual of what the process is to take of the manifold/catalytic converter and replace the gasket. Could I use an oil diffuser for smoke to put in exhaust as a smoke machine to find leak or use a shop vac air blower part to exhaust on muffler to pressure the exhaust? And spray foamy soap to check for bubbles?
Any suggestions would be greatly appreciated and removal process of exhaust manifold, thanks