Ticks Pops and Thumps from only one Channel

I'm hoping you guys can help me with this one.  I have been noticing ticks and pops coming from only my left speaker while the right speaker is noise free.  Switching the phono cables switches the noise to the other speaker.  When I put on  a test record and listened to a dead portion of it, a thump came on about once a second but only from the left speaker.  Removing the antiskate had no effect.  Changing to different phono cables also had no effect. The turntable is grounded to my phono.  Could it be my cartridge (Audio Technica Art 9)?  My turntable is a VPI Aries 3.

Thank you for your input.
Jon