Peavey Rage 158 popping.

[Reponator]

Hello. I'm brand new to electric guitars and amplifiers. I purchase this amp used and all seems to work properly. The popping happens when I touch either the connector, strings, bridge, and several other parts of the guitar. I hook the guitar and cable up to another amplifier and the popping isn't there. Also if I hook the cable up to either phones or cd/tape and touch the plug it will make the popping noise. It does it in the clean and lead modes and it gets louder and softer with the volume. Any helpful hints or troubleshooting advice would be appreciate. Thanks.