loud humming sound at low speeds after highway driving?

I have a 2004 Santa Fe with 130,000 miles on it. This summer I drove it from Cali to Texas and back. No problems there but on the way back after hours of highway driving (avg 80mph) when I would take it off the highway the car would make a lot of noise, humming perhaps, at low speeds.

The sound doesn't get worse with turning and does not occur at idling. The sound occurs with acceleration, like it is working way too hard to accelerate to such low speeds. Additionally, once I take my foot off the pedal it doesn't immediately stop but takes several seconds seemingly proportionate to the decrease in speed. Additionally, after parking it for a few hours, it doesn't make the sound anymore. What is going on? Can I have some advice here? Transmission, engine?

Thanks

 

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