Hi, I have a T4i. At times I get a hissing background noise, especially when I use an xlr cable to hook up to a soundboard when I record events. I even get it at times when I use my wireless lav mic. I switched to manual audio but still had the same problem. Am I supposed to adjust the levels once I switch to manual audio?
Yes, try setting the audio level much lower in the camera itself and sending it a stronger signal. The audio pre-amps in these cameras are not very good so the less they work the better,
With that said, plugging sound directly into the Canon doesn't often yield great results, do some reading on dual audio sound and I think you will find some better solutions for yourself in the long run