As I am sure you can appreciate it is difficult to diagnose a noise one cannnot hear. I will provide an honest goodo faithanswer. I trust this will be appreciated. Please remember that a low rating PUNISHES me for being honest and that I receive nothing for my time until a positive rating is clicked.
the most common cause for this would be an air or exhaust leak, or a failure of the crankcase ventilation system
also, belt drive components can also cause a growling type noise
the next steps will be to run the vehicle briefly without the belts, and if the noise is still present, then have it traced with a mechanic's stethoscope
wheel bearings are also common to growl, but that would be from one wheel, not the engine
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