A phantom noise appeared on the analog out meters of TotalMix, meaning it was not audible. It was more prominent on the right channel. Forgot to take a screenshot but DigiCheck showed that however it occured at around -80 dB, spanning from 0 to around 150 Hz, with a stronger peak at exactly 25 Hz with about -50 dB on both channels (important to note how it was as low as the rest on both 24 and 26). It also didn't disappear by disconnecting the breakout cables.

Figured it must be some sort of mobo radiation or ground loop issue and while tried to troubleshoot it, couldn't locate the source/culprit. Slightly shrugging, continued to use the card as normal in the following days in an "if it'll disappear soon, all the best, if not, then I'll make use with what I have" manner (for comparison rendered a few sounds I did before it happening to know if the phantom noise had actually been added but it wasn't the case).

After two days and a pressing of the button on my keyboard that triggers Winamp's play function (yes, some of us are still loyal fans), there was no sound. I immediately opened TotalMix only to have my pupils dilated at the dismay of the peak-party going on. There was a random 'dancing' of that phantom noise on literally every channel, with a different 'rhythm' and intensity on each one. Each being left and right as well.
Shut down, PSU power off, conditioner power off, ground myself (who knows by this time), open PC case, take out card. ... No visible damage. After some consideration inserted it to my other PCI slot - same thing.

After some foruming and googling it was pointed out that either the card died by itself or the motherboard is failing and caused it. However couldn't find any faulty and bent-ouf-of-its-place capacitors and there were no signs from other hardware nor the behaviour of my OS that pointed to a failing motherboard. Still unconvinced, since after another two days of letting the card 'rest' put it back in to find it working, albeit with the phantom noise only appearing on the analog outs and at the same intensity it did before. In spite of this, it worked for about 4 hours (note that yet again it was another press on Winamp's play that 'killed' it).

The card started its life in my PC in December of 2010, bought new, used relatively frequently (however my guess is that it's one of the first hardware revisions from between 2003-2005 as the feedback card from RME in the box didn't include Vista as a possible OS yet, not to mention 7 - though that might be an overlook from the company and probably barely has an effect on why the card failed so 'early').
The motherboard (GA-MA770-US3 2.0 - put into work in 2009) still has at least 3 years left from its alleged minimal lifetime (90000+ operating hours).

I need to confirm what was/is at fault here as I intend to buy another HDSP 9632 (or HDSPe AIO) but wouldn't want to put it in an unsafe mobo.

The firmware was last updated in December and absolutely no changes have been made in the system apart from one (other than having to do regular defrags with Ultimate Defrag 4 for some time prior - but not on the disk where the drivers are located), which perhaps is best to share: only a day before it happened, I replaced my Avid Pro Tools 12.4 with a cracked version of 12.5 since none of the (legally owned!) plugins worked in it (common error), and all of them ran in 12.5 with even faster load times (just PT being good old PT). The name of the AVID audio driver seems to differ from the previous one (Avid HD Driver (x64) but I'm sure it was called something else before).

In case it -was- the swan song of my card and since I could barely find anything about this online, I'll include some keywords below for people having the same problem (in the future) to let them know that the phantom noise is the card's way of saying it has but days left of its life.

phantom silent fake noise - signal - meter - bounce - random - no audio - TotalMix

p.s.: I couldn't test it in other PCI slotted motherboards since couldn't find any in my hometown, none of the few repair shops had any.