The "rumble" I was referring to is the air supply turning on to adjust the suspension. Like if you were loading something in the cargo area that was heavy enough to lower the rear suspension and create an inclination. Depending on when the last time the vehicle was started...you'd either hear the "brrrrrrrr" (rumble sound) of the compressor kicking in soon after loading...or at least when you turned the ignition on.

If the level sensor is faulting (since you say it was mentioned after you ran diagnostics)...the air supply could run more than normal in an attempt to keep the rear left side level. If this is the case...you'd want to handle this before burning out the air compressor which will increase the cost of the repair. The air supply is there to adjust/compensate when load conditions change etc...but it's not supposed to constantly run.

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