Also, I can play through other channels. As long as the MAIN volume is not very high the pulsing is not audible.

But the existence of the pulsing at higher volumes, and the fact that the mixer won't connect to my computer remain significant problems.

Hi
I have exactly same issue playing from SD card (channel 7/8 on my AR8)
Recording on SD card is clean, checked separately. Issue is on playing, not recording.
All other channels muted
Any suggestion is welcome.

I have had the same problem on my AR16, which I describe as popping in the control room output. It is only there when 17/18 is on and the USB/SD button is set to select USB. It does not need to be connected to USB for the popping to be present. Audio from other channels still passes to control room as it should. Mixer was purchased December 2017 and the problem developed suddenly in January of 2019. Mixer was sent to Sweetwater (dealer) in February. They sent it to PreSonus and PreSonus was unable to fix it. Under their two year warranty Sweetwater replaced the unit. It has been fin since then, until this morning (7/8/20) when the original problem suddenly came back. I will be contacting Sweetwater this evening and if they can't help I'll be purchasing a new mixer, made by someone else.

I have a theory which can explain how a problem like that can develop over time, then to return even after equipment was replaced. It has to do with setup routines and wall warts or ESD (or both).

Depending on your setup routine and location not all gear is grounded before it's connected together. Gear powered from external power supplies (e.g. laptops) may not have a ground connection at all. Due to static electricity and the way ground isolated power supplies work the the various units can be at different voltage levels before the cables between them are connected. On occasion I've even seen small sparks fly when making connections.

This is still not a problem when it happens occasionally. Inputs and outputs are fitted with capacitors to protect the electronics from damage. The capacitors take the hit and, slightly worse for wear, live on to fight another day. Capacitors on audio inputs/outputs can easily outlive the unit this way.

But on data ports the protective capacitors have to be smaller or they filter out the high frequency data signals too. These capacitors can take hits but not as many as the caps on audio sockets. And once the capacitors go the electronics are next…

So with this in mind check your setup routines: Do you set up in a particular order every time? And in the same venue? Have a critical look at anything repetitive, with ESD (rubber soles and carpets!) and ‘floating’ equipment in mind. And take extra care of the data connectors as they are out of necessity the weakest point.

Thanks for that information. Unfortunately it doesn't help my situation. My AR16 is in my home studio and is always left attached to the other equipment in the studio. The AR 16 does not have a wall wart, it has a power supply (I suspect it is a switching supply) built into the unit. It is connected to AC mains via a standard three conductor AC power cord.