01 Taurus. 150k. Car seems to ride fine but at 55mph if I go thru dip in road I can feel a humming/vibration thru chassis. Level road, no sensation. If I put 2 passengers in car it kinda feels like car is laboring at all times. Acceleration seems poor, road noise and general nvh seems high. I get up to speed and if I let off throttle car seems to settle down. Don’t know if it is torque convertor shudder due to extra load in car? Single driver, car seems to have no symptoms. Bad tire? Separated cord? Or trans issue?
It could be any of the things you mention, or none. At 150K and 15 years, the engine mounts have sagged, the exhaust hangers have sagged, the engine doesn’t have has much power as it used to, ect.
Running through a dip shoves the engine downward against the mounts messing up driveline angles and isolation for a moment. That could easily cause a vibe and buzz. Cars can get a little sloppy and weak as they get old. (don’t we all? 
)
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+1 to mustangman’s post.
I’ll add that at that age it would be a good idea to get a thorough inspection of the undercarriage done by a competent shop. It might be rotting out underneath and the unibody losing its structural integrity.
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It will be forsale in next few weeks. $700-800 range.