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Hello, did you replace spark plug wires as well?
Does the overdrive light flash at all or check engine light?
It could be caused by a vacuum leak should be present at idle as well.
I would clean the throttle body very good and once that is done disconnect the battery for ten minutes and drive the vehicle. This should correct the concern for you.
If the hissing noise only lasts a couple seconds that can be normal. If it was a vacuum leak it would hiss the entire time except when you shut the engine off.