Carl Martin Classic Chorus, grounding problem?

[ZeusMalt]

Cm classic chorus hisses when on. Its the older one, not the new square one.

If i put an od-pedal ON before it the hiss is gone. Hiss occures when its powered with 1spot or battery.
This guy had the same kinda thing going on in 2009:

http://www.guitarmasterclass.net/guitar_forum/index.php?showtopic=31279

Is there a known grounding problem with it?