The backspace key on most keyboards has a very distinct sound. Not to mention it's positioning means timing will be noticeably different as well. Might be better off mixing in a few meaningless modifier keys (press Ctrl, Alt or Caps Lock).
I think one can tend to create similar gibberish over time. I've worked on a system where I needed to do a new signup every time I wanted to test a feature and I've run into issues where the gibberish I entered matched an account that I had previously created.