AC leaks to laptop, cellphone surface. Feel by finger!
Yes, this is a normal effect.  It used to be a lot more common than it is now.  On old tube equipment it was common to tie the chassis to one of the power wires via a large resistor.  The resistor was large enough that you wouldn't get hurt by the current even if you were well grounded and touched the chassis.  Usually you could barely feel it.

One side effect of this is the symptom you describe, depending on which way around the unit was plugged in.  When you are touching something conductive that electrified but with a thin layer of insulation (like paint), you become one plate of a capacitor.  As the voltage on this capacitor goes up and down with the power line, the force between the plates changes too.  This is what you feel as varying friction when you gently slide your hand accross it.  Back in grade school and high school I used to have a shortwave radio like that.

Modern appliances are either explicitly grounded or well insulated.  You are probably touching the external insulation with live line voltage on the other side of it, thereby creating the capacitor.