Nissan Repair: I am hearing a noise somewhere the transmission side., nissan maxima se, mechanic shop

Question
Hi there

       Few months ago I had a check engine lights
on.I didn't know what was,I decide to take to mechanic shop.The mechanic had a code reader and installed to the car.The code was  p0420 catalyst.It said something like that.I canna forgot.Anyhow the mechanic said the rear  oxygen sensor was not working.He told me he is going to cut another old oxygen sensor and put it in my 2001 nissan maxima se.I refuse it to be done, But he told me he have done many times in other cars.Finally I accepted.

He put it on.I took the car drove about five miles it started to skip idle speed.At the speed from 10 to 35 mph the car it accelarate by itself.It makes weired sound at that speed..It's like whistling sound.It doesn't go that fast as it use to go.The car was not working properly.The second gear of the transmission was not working properly.I went to the nissan dealer I bought new oxygen sensor  went back to the mechanic.I told him the problem.He said it nothing to do with sensor.

Please tell me what is the problem.I Don't what I need to do.Please send me a diagram or instruction so I can fix myself.

Sorry for the long story.
appreciated that

Ibrahim

Answer
Take the car to a different mechanic and have the new oxygen sensor installed.  Then have the computer reset by use of a code scanner or just disconnect the battery for a few minutes.  Then, you should know if the job by the first mechanic was defective.  From your description I think the exhaust is leaking or you have an intake leak like the inlet hose is cracked.  Check the hose from the throttle body to the air meter.  These get old and crack.  Unmetered air will cause problems with the running of the engine and will also make this sound of a whistle.  Lastly, write off the first mechanic and shop...