What causes that pulsating sound on my PC?

My Rega P3 TT is hooked up to an NAD phono preamp with its
signal-out wires plugged into a RadioShack stereo-miniplug that goes into my PC. This results in a low-volume pulsating
tone (not hum) from the PC speakers which stops when I disconnect the TT. My Cassette deck does not replicate this sound. TT does not have a ground wire, and the NAD has no ground terminal. Could it be the cheapo Stereo miniplug? I'm still experimenting with recording LPs to HD and so far the results have been pretty good. I've not had the pulsating tone on the recordings, but I'm curious what could be causing it. Thanks for your suggestions.
(I know some of you will recommend a CD recorder, but I'm trying this out first because it's more cost effective.)
Can you tell the frequency of the signal. If the needle is not down on a record I'd bet it's table rumble. It could also be LF feedback. You can check that by turning the volume all the way down if you're listening with the speakers connected while capturing. Most likely it's rumble.