"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.