[Mondial]

Is this a transformerless AC-DC radio? If so, then there is a bypass cap connected from the chassis to circuit ground which is also one side of the AC line. When the AC cord is plugged in so that the cap is connected to the neutral of the outlet, then there is no AC voltage on the chassis. When the plug is reversed, then the cap is connected to the hot side of the line and a small AC voltage appears on the chassis which is what you are feeling.

 Provided you replaced the cap with a safety rated cap there should be no problem, although it is always better to have the neutral side of the line bypassed to the chassis.