I have a 1998 chevy Tahoe 5.7vortec 4dr 4wd. Whenever I go into drive and hit the gas i get a whistling sound. it won't do it if i rev the engine in park or neutral. The engine also bogs down as soon as I hit 40mph. I initially thought it was a vacuum leak(intake manifold gasket) It was running pretty bad and throwing codes for the fuel mixture being too rich and random misfires. I replaced the fuel pressure regulator and intake mani gasket. That seems to have solved the rich conditions and misfires but not the whistling and bogging down part. I'm steering towards clogged cats but I just want to be sure before I go cutting up my exhaust. Any input would help me alot. Thanks