Determining the Source of Noise when TT is On


I can use a hand.  I have a scratchy noise coming out of my left front speaker whenever I have my TT on.  The sound is on without the TT turning.  The noise started about a month ago long after I had installed the component parts.  The TT is grounded.  I have a relatively new phono pre-amp MP 1100 with balanced cables going to a 5 yo MX 121 a/v control.  When I disconnect the left cable from the back of the phono pre-amp the noise stops.  Q: how can I determine if the defect is in the phono pre-amp or the a/v controler?  
normantaylor
Excellent guidance.  Thank you!

Doesn't the MX121 have a Phono out.. It's a Mcintosh too. My MX120, and MX150, both had Phono out.. Pretty sure a MX121 has it ..
Every Mac pre I have has at least one and  the old valve units have 2. C11,20,22,C2500,MX110 all had 2, MM and MC phono out. 
MX120, 150 have one..But no way to set the gain..C2500 I can.

Just a thought..

Regards
I am sure you mean Phono input on the MX150 or MX120.
From Turntable to phono input on MX
you may be dealing with something that was 'borderline' ok, and now problematic.  

only phono input? 

as mentioned, move noisy side left cable to right input, see if noise moves with it. if so, problem is prior to pre-amp.

if noise stays left, problem is not tt or cable, problem is preamp.

after that fundamental answer, we move on. if prior to preamp, don't forget the wires/connectors onto the cartridge pins.

if preamp, switch tubes l/r, noise move?

I have one tube socket that very occasionally makes noise, just the slightest tap of the tube, noise gone for months, a loose pin sleeve in the tube socket I believe. so switching tubes, the problem might disappear, then reappear.

easy enough to move a set of non-problematic cables to the phono temporarily, and temporarily run a separate wire for the phono ground, just to rule the cable out.
I did the cable test switching connections in the back of the av control.  The noise changed speakers.  I already put a call into my dealer from whom I purchased the balanced cable a few months ago.  Thanks for the help and direction gents.  You all saved me major headaches and $.  Great forum!