1992 190E 2.3 Exhaust/Noise Problem

Hey Guys...

Have a problem with my 190E 2.3.
To give some history: When the car had 150 000km I had to replace the head gasket due to oil in the coolant. My head was sent out to be refurbished.
The rebuild shop, sandblasted it, replaced some burned out exhaust valves, and shaved the head. My mechanic put it back on and ever since then I have a large amount of noise in the car (i.e. low moaning/droaning noise). The noise comes when I go on the highway. When I reach 120-130 km/hr on the highway and let my foot off the gas, there is a large moaning sound throughout the cabin (makes you deaf in the car). Sounds as if there is too much back pressure which is causing all the noise in the cabin...
I had an original exhaust on the car, and part of it was starting to go. I also went to the dealer and he also informed me that my catalytic converter is bad as well.
Then I cut the entire exhaust off, and put on a universal catalytic converter, and put on a stainless exhaust consisting of stainless pipes with a center and rear muffler...
In the end after changing the entire exhaust, I have found that I get the same amount of noise in the cabin regardless.

Not sure where to go from here... Anyone else have this problem?? Not sure if this is a head problem due to the fact that it was shaved???

Any advice would be greatly appreciated...

Regards,
Jack M