2003 Ford Ranger Whistleing

This sounds like the exact same problem I had with my 2003 Ranger. Does the sound continue while driving? It started off with an annoying electronic sounding whistle with no loss of power. Eventually it will lead to loss of power. I had to take mine to a mechanic before it could be diagnosed. The source was a mostly clogged Catalytic Converter. Instead of stalling the engine it was allowing air pressure to escape thru the exhaust pipes near the engine. The only way to see if this is the problem is to drill a small hole in the pipe between each converter (there are 6 total) and insert a pressure guage and test for pressure. Start at the last converter and work toward the engine. After each pressure check insert a bolt that will block the air from exiting the drilled hole. If the pressure is too high in any area then the converter needs replaced. Has to be done at Ford because your warranty is still in effect untill 75000 miles. If warranty is void then the cost is 2300 or more to fix.