Equi=Core Hum & Other Issues

I’ve owned a Core Power Tech Equi=Core 1800 for 6 months. Recently the transformer has started to hum inside the chassis. It happens when components are on and also when sources are powered down. The transformer hum is intermittent. It ranges from multiple times per day to 3 or 4 days without any hum. I never had DC on my dedicated lines before, so I picked up a Klein Tools AC/DC tester... https://www.kleintools.com/catalog/electrical-testers/electronic-acdc-voltage-tester-12-240v-ac-15-2...
and when plugged into wall receptacles it indicated overload. Line voltage is steady at 122VAC and always has been.
What am I doing wrong, and how should I test for DC on the power line?

I discovered another problem when testing the Equi=Core duplex receptacles; they indicate inverted polarity. All house outlets measure correct polarity.
I’m looking for some assistance in diagnosing this hum. I’m not sure if it’s due to DC offset or if the torroid itself is vibrating in its mounting.

I know this is becoming a long thread, but I must mention that the Equi=Core now trips the breaker at the panel when powered on. It trips several times in a row, eventually allowing power-up. This is a 20A dedicated line from a subpanel which I’ve had for years.
Any advice is much appreciated.

I just have a balance transformer installed in my listening room at the third floor of my house, where run a separated line from the main inlet. Whenever I turn on my pow-amp, the transformer causes a "buzz" but nothing wrong happened, I enjoy the music perfectly. One day I heard a hum noise very noticable, stop for a while, and again. I go around to check and detect that hum noise caused by the hairdryer of my wife from the first floor. Now I know every single time when she use her hairdryer.
Another, balance transformer have two "live" polarity, there is no "neutral" polarity any more.
Well, routers usually are on 24/7, and the power requirements are minimal, so they don't usually draw enough, or cause enough DC offset to cause this.

Again, strongly suggest just turn off every single breaker except the one your conditioner is on.

Call me at 770-667-5633 to discuss your problem. We have seen it multiple times and in virtually every case it is DC on your line. You can call your power company and they can remove it or use our Deep=Core that removes virtually all DC on the line.