There's an article here that describes what filtering is. Linky!
I like this article because it explains what filtering is but it also explains how sometimes "filter" words are necessary. You see, I don't subscribe to the all or nothing rules of writing theory. If you write deep POV, sometimes the only way for a reader to learn something is because the main character heard it or saw it. Sometimes the writer deliberately chooses to filter information because the POV character is unreliable, possibly lying to himself and others. Or sometimes the author just wants you to know that you as the reader and seeing what the character sees not, just some random, disconnected act.
That being said, I do think that an author needs to examine the instances of the words heard and saw and felt, just to see if they're necessary. Sometimes we find we don't need them at all.
Then again, sometimes we really fell like we do (grins)