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?  
Is the unit tubed?
If so there is your answer. Switch first the phono cables to eliminate that, then swap tubes left and right.
I would not know how to check for a cable problem.  I have wiggled connections and it did not make a difference.

Yes, the MC 1100 phono pre-amp has tubes.  I'll give that a try.  I will also switch the cables to rule that out.  Thank you.  
When I disconnect the left cable from the back of the phono pre-amp the noise stops.
If this is the case, swap this cable with the one for the right channel. Does the problem move? If yes, the electronics are off the hook- and investigating the cable itself is a good idea. There are other kinds of defects that have nothing to do with the connectors.

To sort out if its the cable, swap it left for right at the turntable end. Do not swap at the preamp end. Does the problem move? If no=> its the cable if yes=> its in the turntable.
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..

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!
I am sure you mean Phono input on the MX150 or MX120.
From Turntable to phono input on MX

Yes thank you.. gray9hound. That would be a tough one.
I need to pay attention..LOL Tape out (loop) 

MX121 has a place for a balanced input also, noise cancellation cable. Some people use balanced. I don't think phono carts are balanced so
I can't see a reason, unless it is the cable  I'm not an expert on LP stuff, but I've made a few TTs and arms.

Your not a battery powered guy, floating everything, ground,pos,ect..?

Mac pres have Phono in, most have MM and MC, C2500 the MC gain is adjustable, via menu option. 

happy hunting