Exhuast Noise??? Options

[TRDCelicaST]

I had a custom cat-back put on about 2 weeks ago, and instead of going with a magnaflow muffler I went with one the owner of the shop had suggested (Arvin). Although I've never heard of Arvin he insisted it was very similar to the magnaflow. My problem is when the car is almost at WOT I hear a loud clicking noise coming from the muffler but the sound only shows up around 3,000 RPM and then fades out a little after 4,000 and only when the car is close to WOT. I talked to the guy at the shop and even had him ride in the car with me. He says that it's because it's a 4 cyl. motor, but I've never heard this sound come out of any 4 cyl. before. He says if I put a resonator on, that most of the clicking will go away. But it's not a sure thing. I'm kinda stuck here and am wondering if I should just get the magnaflow ($130) or have him put the resonator ($25) on. Has anyone had this kind of problem before? What do you guys think I should do about this? I just dont want to have the resonator on and still have this annoying noise coming from my car and then have to buy a new muffler too.