99 Tahoe - Sluggish

Hey all, had success here before, so thought I'd throw this out.
My 99 Tahoe has been running real sluggish lately. Seems like no hp when I hit the gas, and now it sounds like there's an exhaust leak sound, a bit of a ticking sound. I've done all the usual stuff, fuel filter, check O2 sensor, etc. Now the "service engine soon" light is coming on and when I hooked it up last week, it's not showing any codes or anything wrong. I'm totally baffled, any suggestions? I'm bringing it in to my mechanic next week, but I'd sure like to know what to tell him so I don't run the gammet on the $$ checklist of things to replace before we find the problem.
Thanks guys.
Moon

I had a similar ticking on my '95 Chevy truck that ended up being a bad spark plug wire grounding on the engine. I don't know how much it hurt the performance of the truck though, but it's worth checking.

If your Tahoe is like mine ( '99 with over 100k miles), I would go ahead and replace the plug wires anyway, either with OEM wires or high end aftermarket wires (not cheap Autozone wires).
Richard