Q: Loud metal sounding noise at 20 mph then again at 40-50 mph.

It’s great that you are trying to be your own detective! This information is very helpful for a mechanic who isn’t there to hear the noise. My first thought after seeing the specific speeds was that this noise is transmission related. The Check Engine Light could be related to your problem, and filling the tank was just a coincidence. Or, the Check Engine Light is related to filling the tank and is not related to the noise. Have the computer scanned by mechanic to see what the code is.

I found a technical bulletin for your car about a shudder, or a exhaust moan at 40 to 45 mph. I’ve heard noises like this before. It’s a resonating noise that can fill the cabin and drown out everything else. It happens at low engine speed, and light throttle application. It is related to the "transmission converter clutch" or TCC. If this sounds like the problem you are having, the fix is an engine computer (PCM) reprogram. It modifies the shifting strategy so it does not "lug" the engine. Your transmission fluid may need to be flushed as well if the reprogram does not entirely repair it. The bulletin number is 06-07-30-30A. Only a Cadillac or GM dealer can reprogram the PCM. If you have an extended warranty, this should be covered. If not, you will have to pay for it, so make a few calls as prices can vary.

If you find that this is not the source of the noise, your best option is to have a mechanic ride in the car and inspect the car noise. If you would like to have an accurate diagnosis before taking the car to the dealer, consider YourMechanic, as a certified mechanic can come to your home or office to determine what repairs are needed.