What is AVCC for on the ATMEGA328?

Sorry for resurrecting the thread, I have connected AREF And AVCC pins to Vcc.
And have made a synth with dual pwm audio output, when MIDI INPUT is received it makes a crackle noise.

Here is the schematic:

It's very thin but it's there at the output, also is most noticeable at those 2 "green font markers with arrows" above

Could it be AREF and AVCC reason??? I tryed to RC low pass filtering them but it didn't do anything.
Thanks!