One possible problem is that your inverter isn't hooked up well. If you're using pluging into the car's original wire harness, that might be the problem. Try running power directly from the battery (don't forget a fuse), and find really good ground.
I just bought a ground loop isolator. Will it make my subs sound like **** now? I haven't tested it yet. I just figured it was a good thing to buy.
I had the same problem. I inserted a 3wire-to-2wire AC plug adapter (the kind that gets rid of the ground wire) on the AC output of my DC/AC inverter (which powers the "brick" that feeds regulated +12v to my carputer) and MOST of the annoying buzz went away...... I tried re-connecting the 3rd (AC ground) wire to the chassis, but it made no difference. Go figure.
A "ground Loop" can also be cause my voltage differantals between outputs (most common in multi amp systems) I had one till I rebalanced all the amp inputs to be the same. you can also use a number of high end ground loop isolaters Ie the one from audio source that has settings for floating input/outputs and sub sonic filters. but if you are running more then one amp or a computer into an EQ into more then one amp then I strongly suggest you check all your inputs
Start with the amp gain at Zero, turn the Computer volume to max then bump the amp gain till you have the volume you want and see if you still have noise.. repete for all the other gains,
When I was running a Head Unit into a Pioneer DEQ 9000 into 6 amps it took forever to balance all the input but when I was done I have ZERO noise and 142 DB (more then loud enough)
You will want to make sure that your invertor, amps, and the casing of your computer/power supply are all grounded to the same point on the chassis. I can almost guarantee that once this is done, you will eliminate most, if not all, of the buzzing/alternator whine.