Strange thingie in power cord of used Adcom amp

I just bought a used Adcom GFA 6000 5-channel amp. A small segment of the power cord has been taped into a small loop about an inch and a half in diameter. Inside this loop there is a small, open plastic box containing an even smaller, hollow half-cylindrical piece of metal. The metal looks like a permanent magnet, but does not exert any noticeable magnetic field (it does not attract a nail). The power cord itself is intact with no splices or breaks. The electrical tape simply holds it in a loop around the small metal object.

The seller of the amp tells me that it is a "tloroid" (typo for toroid?) and that it "came that way." He says it reduces interference and noise. To test the amp, I plugged it directly into the wall outlet with no protection or conditioning. All five channels actually sound pretty good driving a KEF 104.2 -- there is no hum or buzz, and only a barely audible hiss with no source playing.

What is this mysterious gadget? Should I leave it in place, or remove it? When the amp goes into my system, the power cord will be plugged into a Monster PowerCenter HTS850.

Thank you.