Power transient heard within the M8

At least a couple of years ago, I bought an M8 for my Dad. For some time now (maybe since last summer), he's hearing a random transient thump coming from it. I've heard it myself over the holidays. It is short-lived (lasts less than half a second, probably less than 1/4 sec.), and only happens once in a while. Maybe between once a week to a few times per day. I can't tell if it is coming from the driver or an internal component. I am leaning towards the driver. Any ideas what the problem is? I'm quite confident it is not in the source material. It doesn't matter which source is selected.

This could be DC on the mains. I had exactly what you are describing happen in my setup roughly 2 years ago.

It happened whenever my wife would turn on a box fan we had in the window - as it was in another room, and seemed completely random, it took awhile to figure out the cause. It would send about 100mV of DC over the AC mains (that registered and disappeared so quick on my spare meter, you'd think it was just a fluctuation on your DMM) and I would get a nice quick thump through the driver. Most people think Surge protectors and power conditioners filter out DC, because they will list AC Filtering, but 99.99% of them don't filter out DC.

I'd start by removing the RCA cable at the back of the subwoofer. If it still happens, with no connection from upstream and the subwoofer on, my inclination would be DC.

If DC, you'll need to find out what is causing this (usually equipment with a motor that kicks on - fridge, compressor, hair dryers, etc.) and replace it or depending upon the price of the equipment (I had a customer's expensive meat freezer do the same), buy a DC filter.

Needless to say, I bought a new box fan for ~$30 as it was cheaper than the alternative.

Thanks for the thorough reply. I passed on the instructions, and we'll see what happens. I told him to give it at least a week before giving up on it being external DC.

After about 13 days of no noises with it always on, and the RCA cable disconnected, I had him reconnect the RCA cable, and leave it in the always on switch position. It seems the DC transient is not the culprit here. So we'll see if it is still making the noise. If so, it is somewhere upstream.

I had a problem like that a while back. The sub would just give out a random thump every now and again. As I remember the problem was noise coupling onto the coax cable. I don't remember if the solution was a higher quality coax cable, rerouting the cable or both.

Try rerouting the cable first making sure it's not next to any power cords or speaker wires.

If that doesn't do it then it might be some weird ground loop. I've had ground loop problems with other subs but not with my Outlaw.

Forgot to post my final results from that check. Leaving it in the always-on position did solve the problem. No transients/thumps since. The other position, whatever it is called where it comes on when signal is there, is where the thump would show up.