Cubase 4.5.2 - Static using Channel Monitoring?

Has anyone else experienced this?

I am re-recording tracks for a song written some time ago. In order to hear my playing in the context of the original song (and because I am familiar with the sound of the original song) I turned on the track monitoring to get all of the inserts and EQ.

Now I can distinctly hear some static in the mix that wasn’t there before. I know for a fact that it isn’t my cables because, using the same signal path, I recorded other tracks for this song and one other that did not use the monitoring feature and have no static as a result.

Am I doing something wrong? Is this a known issue? Etc.