exhaust problems

Okay, i ditched the stock muffler for a straight through style with an adjustable silencer on my SL2. It's not a very expensive one because i'm trying to save my pennies for a Thermal R&D cat-back system. Anyways, it sounds like shit. I've played with the silencer and I can't seem to tune out this high pitched metalic rasping sound that occurs after about 4,000rpm. From a dead start it sounds great, deep and throaty, but once the rpm's climb over 4k it all turns to crap. Someone told me that that's how it'll sound for a while until it gets broke in. Is this true? I've had it on the car about 5 days and it doesn't sound much better than it did when i first put it on. Did i just get stuck with a junk muffler? Somebody help.