Does ADC in + PWM out = trouble?

Other Hardware Science and Measurement
10

So I ran this:-

void setup() {
 Serial.begin(19200);
 Serial.println("Running");
 analogWrite(3, 75);         <---  THIS STATEMENT TOGGLED ON /OFF
}

void loop() {
 Serial.println(analogRead(4));
}

on a Nano just floating on my bench. Nothing connected to any pins.

Serial Monitor shows analogue readings of 18X with the analogWrite() statement commented out, and 19X with the PWM running. X is a changing digit. This is less effect than I expected, but statistically visible. You can clearly see the 50Hz mains hum on Serial Plotter. Hmm. It may not be an issue then with buffered input and output.