Humming noise at slow speeds.

Hello, you did not indicate which engine your truck has installed in it. This is one factor I would appreciate learning to help give advice. But beyond that to be honest with you, noise is really should be detected in person by somebody with a train deer who knows what the noise may be in person. I appreciate your description, but I can't tell whether it's vibration and resonance off of a frame piece, internal engine noise, intake manifold noise, catalytic converter heat shield noise, or anything else. It's too difficult to determine. So, I'd suggest visiting a local shop near you and showing a technician your noise first hand to get impressions. https://repairpal.com/auto-repair-near-me