Credit due to the Subaruvanagon list  -  to test your exhaust system for
leaks set your shop vac to blow instead of suck. With the exhaust system
cool, tape the Vac hose to the tailpipe, turn on the machine. With the
exhaust pipe pressurized, and using a spray bottle filled with a dish
detergent mix, start at the manifolds and spray every connection, all the
way to the rear. Chances are that you will get bubbles being blown at the
leaks. Many can be fixed simply by retightening fasteners. Others require
new gaskets, new pipes, etc. But it's worth the try.