i recently installed a header and downpipe and now i am hearing a whistling noise almost like a turbo but not really.i dont know if this is where the o2 sensor is suppose to go in because it is not sealed well enough or what.i installed a test pipe and put the o2 sensor from the cat into and of course the CEL came on.the o2 sensor from the engine is attached to the header but the one on the downpipe might be causing the whistling noise,is it that or what?help please its on a 2002 4ycl accord se

 

double check your install and check it for leaks. did you use a new gasket?

 

yes i used the new gaskets that came with the header and test pipe there were no gaskets on there before just between the header and the engine

 

An obvious leak somewhere. Possible crack? Be sure the manifold nuts are torqued, did you clean the surfaces and follow the torque pattern? Try some high temp copper silicone around that O2 bung.