This is going to be hard to explain, but lets see.

02 Sierra 186,000 miles 5.3 L - At high speeds the truck seems to perform good but at 65 MPH or greater the pitch in the engine changes as if it bogging down. I don't feel any loss of power but the change in pitch is noticeable by everyone riding "even the wife" asked about it. Sounds like a leak in the exhaust she says. When pressure is put on the motor you hear it at 65 or higher, but when released it goes away until you accelerate again above 65. When the speed drops below 65 and traveling at 55 and accelerate it doesn't happen, only at 65 or greater. This just starting to happen in the last 7,000 miles or so.

Any idea'*?

does the rpm change with the noise?

No, it doesn't appear so.

Try duplicating the noise by powerbraking the truck to narrow down the noise. With that many miles, motor mounts could be worn, also have seen issues with the transmission dipstick tube hitting the firewall. And seen some tahoes with a bad exhaust pipe moan noise. If its a exhaust leak check for broken manifold bolts at the cylinder head and cracked a exhaust manifold.

Thanks for the help thus far. I did a check for motor mounts by power braking and checking for engine movement, no movement. Crawled underneath and tried to wiggle and move the exhaust, nothing as everything feels tight. Transmission dip tube feels snug.

Here is some more information that may help. About 3 years ago the cadillac converter was replaced as it coded to be plugged, weeks later I removed the heat shield because it was rattling. At times after start up and until things warm up I will get a rattling thumping sound that appears to be coming from underneath. It sounds like heavy metal on metal then goes away usually in about 5 minutes, but only noticeable when moving less then 20 MPH and goes away when idling and not moving. It could be happening at higher speeds but unsure. I can't reproduce this as it is very intermittent. RPM at 65 mph is about 1900 and MPG is 15.5 the last two fill ups. Above 65 mph the motor will begin to roar at a different pitch, but if gas is released it goes away. Then when gas is applied again above 65 it reappears. Unsure if any vibration is occurring, if it is it is very little to know for sure. Tire wear is about 50%.

ls engines commonly have the exhaust issues he listed, the heat sheild for the manifold, manifold gaskets, etc is where i would look first, look for soot escaping, black marks etc. the heat shield on many of them ends up rusted through, you dont really need it

I know you replace the cat a while back, are you sure its still intact on the inside? Have seen some shop throw cheapies on and make noises later in time. Just a thought.