Try this (from the AE help manual) — I just used this technique and it was miraculous at cleaning up bad/noisy greenscreen plates:
Removing grain in frame sequences (6.5)
The Temporal Filtering controls of the Remove Grain effect use a statistical algorithm to blend the current frame with previous and next frames. These controls are particularly effective in removing compression artifacts from DV or video footage. Because temporal filtering works on the basis of differences between frames, it is useful only for sequences.
To properly evaluate the results of this filter, the sequence must be reviewed in real time, either with a RAM preview or by rendering to a file.
To add temporal filtering to a sequence:
Apply the Remove Grain effect to your image. (See Removing grain from an image (6.5).)
Try the following techniques if you want to improve the results:
To quickly reduce the noise of a sequence that has a lot of buzzing noise, set the Noise Reduction value to zero and the Temporal Filtering Amount to 100%, and render the sequence.
To speed up previews, apply temporal filtering to your sequence after all the settings for a single frame have been adjusted.
To retain a layer’s effects and also apply temporal filtering to it, precompose the selected layer (choose Layer > Precompose), and then apply the Remove Grain effect to that layer.
Jim Dodson
Quad 2.5 gig Mac – OSX