Getting a crescendo out of a comb filter over a fixed length of time?

I'm trying to create rolls from streaming audio but can't figure out how to get a crescendo effect out of a comb filter. In other words the subsequent echoes created by the comb filter gradually get louder as opposed to dying away.

I know this idea screams the use of feedback but I can't understand how to limit the feedback to a fixed time, thus creating an envelope that can either be a crescendo or decrescendo for each click....

Thanks,

Rhys