After oil change, sounds like driving in low gear. Why?

What is your question?

Research showed other person had same problem (w/ a Dodge, I believe). The transmission fluid is apparently too low or too high for this to happen. Added fluid; noise (low rumbling hum -- sounds like a truck driving beside you) lessened but still there. Any other possible solution? There's no light to service anything.

What makes this problem better or worse?

Nothing. Noise continues when slowing down till come to full stop.

How long has your 2000 Buick Century had this problem?

about 4 weeks