Vibration from Fan
If your wall oven uses a fan, the fan mechanism could be vibrating to create the growling sound that you hear. Fans are necessary for convection ovens that use air current, a common feature for wall ovens. If you can narrow the noise down to your fan, you may be able to solve the issue by cleaning it or screwing components down more tightly. Fan motors can also make a growling noise, a sign that they are starting to fail.
Fan Blade Issues
Sometimes the fan is working properly, but a convection fan blade has become bent or warped. When this occurs, the fan cannot run without scraping the blade against the fan framework. This can create a deep rattling or growling sound. Unfortunately, you will need to fully replace the fan blade in order to reduce this noise issue.
Sometimes combustion in a gas oven can cause growling or rattling noises. If the oven only makes the noise when it is first firing up to reach the baking temperature, you should stop using the oven and inspect it for any signs of damage. You may have a blockage in your gas lines, or a malfunction that is causing your oven to use more gas than necessary. This could be dangerous, and requires a professional examination.
Sometimes the burners that your oven compartment uses can become dirty, clogged with particles or corroded. When this occurs, the burners may shake or rattle when they are being used. Make sure your burners and their ignitors are clean and properly fastened to the oven. If you cannot see any signs of dirt or damage, calling in a professional is the best route.