Q: Our overdrive just plain sounds weird when on so we leave it off. What't wrong. on 2000 Hyundai Elantra

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The car also don't seem to run right either, so we just leave it off all of the time. We have about 150,000miles on it now. We get anywhere from 20----30 miles per gal. depending on if heat or air is on, or city or highway miles. This is a lot better then our 95 van that it replaced, it only got 9 1/2 mpg.
(1) Answer
You'll need to address these issues one at a time. The Transmission is in need of a specialist to determine whats wrong with the overdrive. By running the car without the overdrive on & functioning, its costing you miles per gallon as the engine is working harder at a higher rpm.
As far as your 'not running right' issue, a proper diagnostic check by a qualified technician might serve you well.