I've been having issues with my early 2013 15" MBP. The problem started after a drink was spilled over it (a perpetual risk in the live event industry...). Whenever I'm using headphones, but before any audio actually starts, there's a noisy hum on the right channel that sounds like interference. The left channel is fine. Whenever audio begins to play, or an app/webpage that uses audio starts, there's a click in both channels as the soundcard kicks in. Once the soundcard is on, there's no interference in the right channel. After a while of not being used, the soundcard disables, and there's another click, and the interference is on the right channel again. The on/off click in the left channel is bearable, but it's so prominent in the right that I think it could cause damage to headphones/speakers.
The level of the interference/click is independent of the volume setting, and it doesn't seem to be heavily affected by the charger being plugged in. It occurs on any set of headphones plugged in to the jack. It doesn't happen when using the internal speakers.
I recorded the output of the right channel, here's the enable and disable clicks, respectively:
As you can see, there's also a slight DC-offset present. This may just be a result of my recording method, but I think it might stem from the same issue. My guess is that the drink shorted out a capacitor used for filtering the output, and thus the nasty unipolar pulse that would normally be removed.
I'm not entirely adverse to opening the case up to take a look, and I'm handy enough with a soldering iron, but I'd rather not chance it. I'm sure a proper repair technician could replace the relevant board fairly trivially, but before I stump up for that, does anyone else have any similar experience?
Edit: Resetting PRAM + SMC makes no difference, nor does turning off Automatic Graphics Switching or Put Hard Disks to Sleep.