I vote for an obstruction or some other problem in the exhaust. Taking away restrictions in the exhaust is a first step in getting a few cheap horsepower added to the engine output. I have never heard of it causing a problem.
Adding a configuration that changes lengths and diameters arbitrarily can change the tuning of the system, resulting in interfering pressure pulses at the entry to the cat or muffler, which can be a net restriction. Same with new bends and paths. And howling is usually the sign of a small hole, or series of small holes that whistle under certain pressure and flow velocity conditions.
Cobbling a system from another car with a different engine offers the potential to cause some problems. Sounds like you need a new exhaust designed for your car. Try www.timevalve.com.
Good luck. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)