I wouldn't be too quick to buy/make/use a silencer, or anything that might be construed to be a silencer. I would be careful to check with law enforcement, legal or not if the sheriff wants to hassle you you're facing trouble, and research the all applicable laws to seal the deal. Defending yourself in court gets to be time consuming and expensive even if you are found to be entirely in the right and are acquitted.

You may find out that having a silencer for your pellet rifle is as involved and expensive as owning a silencer for any regular firearm.

http://www.pyramydair.com/blog/2005/05/what-about-silencer-for-your-airgun.html

http://www.beemans.net/silencers_on_airguns.htm