Resonating Sound w/RPM; Possible Exhaust?

98k, 5.7L with 68,000 miles:

I've got a strange groaning/whining sound that increases in pitch with RPM increases. I've been all over the vehicle with a tube trying to locate the source of the sound. It is getting more pronounced with time and always does it.

It seems to resonate throughout the frame, but I get a sense it's coming from the exhaust. I think I've eliminated any belt driven components (Alt., AC, PS) and I know its not the fuel pump (Though it makes noise like everyone else's)

I've also noticed a slight loss of power, and am getting horible fuel mileage.

Could this be a Cat. starting to plug up? Other Ideas? Or am I way off base?

Have you done any head work recently or put a new motor in with a higher compression? Exhaust work? Does it rattle your cab any. It could also be you cat. Reason I ask about the heads or exhaust is that your exhaust may not be round enough. I believe chevy has 2 and a fourth inch exhaust. May need to go 2 and a half.

It's all stock. I know noises are hard to describe as everyone thinks differently. I've never experienced anything like it. It is not mechanical sounding at all.

My gut is telling me its a restriction somewhere in the exhaust, though I just can't tell for sure.

Does anyone know of an easy way to test for blocked cat? Before I just take it into a shop?

As long as you're POSTIVE the noise is not coming from the motor or tranny then definately suspect the CAT. Don't know of any way to check it. I do know that muffler shops can check them, but don't know how they do it. Could also be the muffler itself???

Another sign of a restricted exhaust is a rise in engine temp under a load. Have you noticed that???

It's been over a month now; did you eventually discover the source of the restriction? There is a way, of course, to check for a restricted CAT. I was just searching the subject of restricted CATS and came across your post.

sounds to me like your rear end is goin out. the reason you wouldnt have detected it when you listened to stuff individually is that the vehicle has to be in motion to make it resonate.

the noise will sound kind of like when you have two sounds at almost the same pitch, but a little bit off, so that there is a eerie "wah wah wah" type of dissonace as the 2 tones fight each other. my truck makes that noise, especially at highway speeds. the resonating seems for me to speed up as the vehicle speed increases. pretty annoying noise lol, but i dont want to change my fluid im too lazy