Short?

"failing cap" refers to a power supply capacitor in your Mac power amp. 

Some appliances, particularly those that draw a lot of power and have motors, will send a noise pulse into your house's AC circuit of your house when they cycle on or off.   (And, if you have central air, you can't move the stereo to the others side of your circuit since they run on 240 volts, using both sides.)

The power supply of your power amp takes AC from the wall socket and turns it into DC at the voltage needed by your amp. Normally, power supplies are designed to filter out most AC noise.  How old is your amp? Large capacitors in power amps often start to fail somewhere around 20+ years, though it can happen before or after that.

Also note, the amp may be fine, and it is a power supply for a source.  You indicated that it is only your turntable straight to the Mac, so that likely isn't the case in your situation.

It might be that the condenser on the AC unit is going, causing a huge current drain and transient when the motor kicks in.  If your Niagra is doing its job, and the spike was generated over the wire, it would have been blocked at the power conditioner, which just leave airborne EMI entering the Mac.

There probably is a "run capacitor" on your a/c compressor. As pointed out in other replies, it is a sign that you might need a service call. If the fault only occurs when the a/c compressor starts and runs, then the trouble shooting should be easy.  Good luck. BTW, this should be a simple service call, perhaps, and I STRESS perhaps, you might be looking at a compressor, but I would get a second opinion for anything over a couple hundred$$$

Power conditioners don't restrict all noise coming from the outlets in the house. My Shunyata Denali allows noise through to my speakers when I click on my vpi record cleaner. Clearly audible. So your Niagara won't stop everything on the line. 

j-wall

Power conditioners don’t restrict all noise coming from the outlets in the house.

A good one should get very, very close.

My Shunyata Denali allows noise through to my speakers when I click on my vpi record cleaner.

What makes you certain that the noise is coming through the AC line?