My 1990 GSX had one problem that I cannot figure out. I am getting this wierd exhaust groan/rattle from 3500 RPM all the way to redline. I originally thought it was my 14b exhaust housing which was cracked, but I replaced it and it still does the same thing. I pulled on the throttle and listened a little closer and it seems to be coming from near the timing belt or right under the timing belt. Anyone have any idea? i'm pretty sure it's exhaust related, but wanted to get some ideas before having my local shop diagnose it.