Sounds like line noise from the PC (assuming you are using a car pc and the Mobo's analoque out connections).
From sound card driver zero all channel outputs except the sub channel. If it is audible and varies with the sound card sub out channel gain then this is the most likely cause.
I had similar issues using the analoque outputs for on Home theater PC the only solution I found was to use the SPDIF digital output directly to the external amp.
In a car the only to remove this is to process the outputs with a DSP module such as the Alpine PXE-H650 are other setups.
Another possibilty would to use an external sound module such as Creative USB Xi-Fi card and feed the Stereo rca oututs into an equalizer that then filters the Sub channel.
So I haven't done anything yet because I haven't had time but the sound is getting worse and a lot more frequent.. Here is another clip:
With it getting worse over time do you think that points to the SUB itself?
Try here and the following suggestions.
There may be other suggestions earlier in that thread.
I'd like to copy and paste, but I just couldn't be bothered.
If I have replied in haste & misinterpreted, I apologise.
So you linked a post in my own thread?
Originally Posted by OldSpark