Turntable Hum

Full disclosure - I hardly know anything about turntables.  I know to plug it into the phono outlet, that having a 33rpm record playing on the 45rpm setting makes everyone sound like Mickey Mouse, and that loud noise induces all sorts of nice vibration-related issues.

Now, my question.  Recently I picked up a used Sansui PD-11 turntable with a relatively new Audio-Technica cartridge on ebay, so my dad could get back into listening to his old set of records.  Showed up, plugged it into his old Sony STR-V3 receiver, and got a record going.  But, lots and lots of volume-relative noise (similar to what one would hear with sensitive headphones on a high-gain amplifier).  Due to some other issues the receiver had, I figured the noise was just due to something inside the unit (I had taken a quick look before powering it up and didn't see any bulged caps, cracked resistors, etc).  Picked up a Sansui 800 receiver to use instead...and...same problem.  On the FM selection, the Sansui receiver is dead-quiet with the volume pot maxed.  Switch to phono, and the noise comes back.

In taking everything back down, I touched the record, and much more noise.  Hmmm.  Touch the metal plate below the record, exponentially more noise.  Culprit found.  Except, I have no idea what exactly is happening to cause the problem.  It's not record noise, since this occurs even when the receiver is set to phono input, and no record is playing.

Any suggestions?

Hm...the original RCAs had an external ground with them, but I had to solder new RCAs in because the original plugs were trashed.  There is a solder pad inside for a ground wire, so I could always run a small ground out and connect it to the outlet screw.  Given where it was on the original RCAs, though, it almost seems like it would have been grounded to the receiver case...but the Sansui receiver only has a two-prong electrical wire (and not even balanced as a line/neutral wire).

Forgot to update after twiddling with the turntable...ground it was.  Ran a wire from the ground location inside the turntable out to the receiver chassis ground, and no more hum.  Figured I'd toss it out there in case anyone did a search and dug this up.

Thanks for the help!