eliminating hiss from carputer?

A ground loop isolator is simply an isolation transformer.  There will be no frequency loss at all.  A PAC SNi-1 will be a good choice to try.  It is a directional device.  There is a dedicated input and output.  The device has a gain level of 1.25 to 1.  If it is connected backwards, it will be a reduction instead of gain. 

When you had an improvement with the power conditioner, it too is an isolation transformer. 

IAAI: Thanks for that info... that's good to know that it won't affect frequency response, and I appreciate the suggestion on which one to look for. Nice and cheap too, which is a plus... Hopefully I should have more than enough suggestions now to get this solved...

Alright, I've got a bit of an update... I've played with a few different caps, but none seemed to really work. Off the top of my head, I think they were 0.1, 0.01, and 0.00047 uF (they didn't have a 0.001 at the time). I didn't solder them in to test, so perhaps that would've changed things, but I just shorted them across the leads to the computer. Still, the same issue, either with or without the caps. However, I bought a cheapo ground-loop isolator from The Source (former Radio Shack), and it worked reasonably well. It's still not perfect, and I've got a bit of interference, but nowhere near what I had previously. I'll likely return this one and look for a better one, probably the one that was recommended above. Unfortunately, due to bills, I haven't had the spare cash to buy a second power supply to test out, but I should be able to try that as well sometime next week. Maybe I'll get lucky and not need a ground-loop isolator at all.

The capacitors you tried will do absolutely nothing as far as trying to filter out your noise.  There is a ground loop, you will need a ground loop isolator.  You can try a new power supply, but I am pretty sure that will not help you.  If you want to try a capacitor, you are going to need a 4700 Microfarad device.  No decimal point in that number. 

Geez Idiot, you're supposed to write that BEFORE the caps are acquired (ie - after I recommended them). Certainly there are times when they help, but it depends on the noise; cabling; layout etc. Maybe mid-sized caps are more successful - like 4,700uF which is around the size of that cap fitted to the IgCoil+ of a friend's car (which I removed with no ill effect - but may I fixed any or contact issue). Maybe its that I have not worked on very modern cars.... Usually large caps (stiffening caps etc) didn't work because of their construction (size, electrolytic etc). (Do they have lower capacitance at high frequencies...???) And the battery is in many ways a large cap... Te caps are usually an easier and cheaper test than a GLI.    But if a ground loop is the problem, caps might help, but it won't fix it. Sorry for the bottom male bovine beast.

I must have read over where you suggested capacitors.  Had I seen it, I would have stopped him from purchasing them. 

Oldspark and IAAI: No harm done with the caps, I decided to go that route initially as I spent a whopping total of... $6 on them? Besides, I was hoping to go with a power supply fix, rather than an audio line fix. In regards to a GLI... it was suggested to me by an electronics shop that I could get a couple transformers and wire them up to the audio leads, and that should work essentially the same as a GLI. I'm not sure if this is the case, but that may be another possibility for me. My DAC uses 1/4" balanced outputs, while my headunit takes a 1/8" stereo input, so right now I've gotta go through a couple adapters with my GLI to get the right plugs. I've got a cable I made up with the appropriate ends, so I wouldn't mind splicing a pair of transformers into there (and grounding them to the chassis), and using that instead. Any thoughts on that? It was suggested I use 600 ohm transformers, although apparently frequency response rolls off below 100 Hz in the ones that were shown me, so I'd rather find something with a little better response.

I would personally avoid the GLI. That's just me. There is (to me, anyway - but I'm a fanatic... LITERALLY) an audible attenuation in the highs, depending on the quality of the device, and the core material, ferrite cores being better at high-frequency transmission than iron cores. The little 600 ohm audio transformers you might be able to get at the shack are lo-fi devices, for sure. They're primarily designed for telecommunications applications... from 100Hz to 2KHz range, really not good for true full-range. If you are already going from balanced connections to unbalanced connections, consider one of these guys... Gut it, go from balanced to unbalanced (and vice versa) AND ground loop isolate in one fell swoop. While reading about your problems, I thought about this as a solution for MY ground loop issues as well. For $60, I see an end to all my alternator whine problems, too! I've used these in the past, and had no real frequency response issues... Small, ferrite cores with few turns on those cores makes for a low-inductance transfer of audio frequencies. These would likely be better for you than a purpose-built GLI.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

I do not like using them either.  I am a firm believer in doing whatever possible to eliminate the ground loop.  I have tried in several systems just like you have.  The only way I have found to get rid of the noise was a PAC SNi-1