Exhaust/noise issue assistance please

[Steward75]

Hello everyone. I own a 1990 Mustang LX Hatchback 4cyl automatic. I will admit my knowledge of cars is somewhat limited but learning as I come across issues how to deal with them. My latest issue is a noise problem I'm having with my car. It seems like the noise inside my car is a lot greater than what it should be. I dont think it was always this loud but then again I've had the car now for 2-3 years.
What had first happened is the tailpipe finally went on my car from rust, so I had to get that replaced. Well then on top of that, upon inspecting my muffler, i noticed that it was rusted out too, so I replaced the muffler. Well, when my father went to order the muffler, he asked me if I wanted a "performance" muffler, I said, if it doesn't cost through the roof, sure. Well, he comes home wiht a performance muffler. I noticed that the muffler didn't quite lineup witht he rest of the exhaust pipe system under my car. He told me he would make it fit and I asked him if this was made for a Mustang and he told me it was the "generic" brand..... (red light, red light went off in my head). I went ahead though and let him put the muffler on my car. After playing with it for a while, he finally made it fit. Although to MAKE it fit, he had to make his own tail pipe and we had the tail pipe end just behind/next to the rear left tire. My car now had a nice purr when you stand on the outside.

UNFORTUNATELY, the noise on the inside of the car is still unbelievablly loud. The noise only occurs when I accelarate. It's like a buzzing noise that gets into your head very easily. Is this a common thing to happen for "performance" mufflers, because the muffler isn't made specifically for a mustang, becuase my car is having a bad "Reaction" with the 4cylinder engine and the muffler, or the fact that the tailpip ends at my rear left tire well? OR could there be a hole right around my catalytic converter?
Any help or suggestions would greatly be appreciated.