Faucet has current
I've recently purchased a foreclosed home and am working on renovations. I was rinsing some paintbrushes in one of the bathtubs recently and noticed a slightly painful sensation when my finger, which had a small cut near the cuticle, touched the water in the tub. At the time my other hand was on the faucet. I shortly after realized that it was a minor electrical shock, and it didn't occur when I took my other hand off the faucet. I got out a multimeter and measured around 5 volts between the faucet and the water in the tub. I've since measured the dry tub, and still see around 3 volts between the faucet and the drain.
I'm no electrician, and I don't intend to attempt electrical work to fix this. But I was wondering if anyone had suggestions about things I might be able to test or do before calling one out to speed up the process or lower the costs. The home is a 1970 brick ranch, in South Carolina. I know of two potential issues, and was hoping to get some feedback.
Issue #1: A new water heater was installed recently. The previous heater was estimated to be 15+ years old (we couldn't find a data plate on it). It also apparently predates the heat pump system in the home, as the air ducts/returns prevented the removal and replacement of the old water heater. The old heater literally cannot be removed without removing ductwork. It was drained, with the water pipes cut and capped, and the electrical wires disconnected. The new heater was tied into the water lines at a different point in the crawlspace, but uses the same electrical wires. The water lines are all copper, but they are attached to PVC intake/outflow elbows on the new water heater, so I wouldn't expect current to flow across the intake and output pipes. I've read that the cold water side of a water heater is sometimes grounded, so thought this might be relevant. I didn't install the heater.
Issue 2: The home inspector indicated that the ground and neutral wires are not separated in the electrical subpanels of the home. I don't know enough about whole house wiring to know if this could contribute, but wanted to mention it.
I'd also point out that I've used the voltmeter to look for current on other faucets and drains in the home, and haven't can't see anything. We obviously aren't using the bathroom in question for anything until we can get this corrected, as any electricity/water combination scares me. Any input about things I could troubleshoot myself before calling for help would be greatly appreciated.