Loud Exhaust even after new "Catback"

[Bobby1989GL]

Hello,
I replaced the exhaust on my 1980 240GL with one of the Walker kits. It needed to be replaced badly. When getting around 3000-4000rpm the exhaust seems to be very loud then quiets down considerably after that. It almost sounds like it's coming from the cat converter below the floorboard. I'm afraid that the cat has gone bad and the loud noise is it being restricted. The power level seems fine and I can't seem to find any leaks from the header pipe down to where I installed the new piping. Also another note it exhibited this behavior before I replaced the exhaust but was a little louder. Any idea's for this Volvo newbie?

Bobby