Short?

+2 for Ralph. Have your unit serviced, they can clean it and check the voltages and wash the coil. That runs about $100, retail on a capacitor is $150-200 installed. Or change it yourself if you can safely. Do it now before you incur further damage to the air unit. I live in a hot area and have 2 large units. I blew a cap on one, and two weeks later on the other. Both days it was *110. One of the caps was only 2 years old. I’ve also encountered fluttering lights when a high current draw kicks on. This was a grounding issue. Depending on the age of your house you can also have tired breakers. I had a breaker for my pool get weak and it was sizzling on the bus bar. I replaced the breaker and had to move it to an unused position as the buss bar was pitted. I’m ok now , but I’m waiting on a new service panel as I have solar coming right after that. So get your air  unit checked and proceed from there. Best of luck , Mike B. 

Do not know if your Mac is hooked up to the Niagara. I had the same issue on a secondary system with a Panamax e5100 surge protector. I disconnected the amp from the Panamax and the popping stopped.

@mojo771 You may not know that Ralph is "atmasphere".

Listen to him. He is the man, man.

Sorry for tangent but since some here have mentioned a failing cap in the amp being responsible, i'm wondering if a similar issue i'm having is responsible for popcorn popping sound in my REL sub?  Only began recently and is occurring while A/C is running, not when it starts up.