You've actually got me stumped. It's pretty easy to get some voltage between different metals due to chemical reactions. That's how ordinary batteries work. However, a single-cell battery typically has 1-2 V, so 5 sounds oddly high. Could there be multiple junction between different metals, effectively making some series voltages? Also, 5V wouldn't be enough to feel a shock. Was that a dc voltage? Maybe there's a larger ac voltage.
As a total amateur, not to be trusted in any way, I'd make the following recommendation.
1. Check the current from the ducts to the ground with an ammeter. Any of the standard sorts of voltage sources couldn't supply much current. If you've got more than a few microamps, I'd be pretty worried about where that voltage is coming from and hire an electrician.
2. Assuming there's no alarming current, ground those ac ducts in several places. That should leave everybody much safer.
(published on 07/24/2012)