You didn't mention the type of background noise you're trying to filter, but anyway,it's one of the features that doesn't work particularly well in my opinion. First you have to make deadly sure that you select the EXACT bit of noise sample to profile, then I found you had to apply the filter in extremely small doses i.e. slide right across to less, apply just a little bit - then save the file (don't try re-applying it over and over). Re-open the file and repeat the process until you're happy with the result.

I must admit, I only use it now when I've used a sample in a track and I get that annoying "pop" you sometimes get when a sample cuts off, even though the sample is perfect. I zoom right in and profile the pop then apply the filter to the exact place I profiled. Works fairly consistently and is non-destructive.