"But it sounds better at night...."

I can affirm that it is not necessarily time of day but the weather and general neighborhood power use conditions that determine if the power will be less or more clean. Today’s power versus pre-internet and especially pre-HVAC was substantially cleaner.

Although my audio systems sound excellent most of the time, an example where spurious buzzing and diminished sound quality occurs every summer in Los Angeles when the temperature is above 110 degrees and A/C is being blasted/universally being operated. Luckily, this lasted about 8 days throughout the summer months and primarily after 4pm until 12am. The entire winter, I enjoyed fabulous sound.

Please note from my profile that I have excellent insulated cabling, a very dense and secure room against RFI/EM, dedicated audio equipment only power subpanel and wiring and a Bryston BIT-20 isolation transformer.  

I was reading up on electronics and was reminded that capacitance and resistance values can change quite a bit depending on the ambient air temperature. Sometimes, it can be enough to knock them out of their rated values/ what the circuit was designed for. Many places are cooler in the evening- from the place that generates the juice all the way to our speakers. Hmm

Coronal hole? C’mon man!